diff --git a/app/assets/javascripts/main.js b/app/assets/javascripts/main.js
index cef79eec273a4004d40c5a4c2970e17e04863293..3ace25ad3fa6ad669576f0b772217d27a080d613 100644
--- a/app/assets/javascripts/main.js
+++ b/app/assets/javascripts/main.js
@@ -75,6 +75,7 @@ import './projects_dropdown';
 import './projects_list';
 import './syntax_highlight';
 import './render_math';
+import './render_mermaid';
 import './render_gfm';
 import './right_sidebar';
 import './search';
diff --git a/app/assets/javascripts/render_gfm.js b/app/assets/javascripts/render_gfm.js
index bcdc0fd67b8ed4923d0fe91bfb4c09337e1580d5..bf6fc0ec30568e2e540e3fdf725daa1cec453e2d 100644
--- a/app/assets/javascripts/render_gfm.js
+++ b/app/assets/javascripts/render_gfm.js
@@ -2,12 +2,13 @@
 
 // Render Gitlab flavoured Markdown
 //
-// Delegates to syntax highlight and render math
+// Delegates to syntax highlight and render math & mermaid diagrams.
 //
 (function() {
   $.fn.renderGFM = function() {
     this.find('.js-syntax-highlight').syntaxHighlight();
     this.find('.js-render-math').renderMath();
+    this.find('.js-render-mermaid').renderMermaid();
     return this;
   };
 
diff --git a/app/assets/javascripts/render_mermaid.js b/app/assets/javascripts/render_mermaid.js
new file mode 100644
index 0000000000000000000000000000000000000000..a253601f8e8f53541d0dc2e7bd9ed69ebf60eac5
--- /dev/null
+++ b/app/assets/javascripts/render_mermaid.js
@@ -0,0 +1,30 @@
+// Renders diagrams and flowcharts from text using Mermaid in any element with the
+// `js-render-mermaid` class.
+//
+// Example markup:
+//
+// <pre class="js-render-mermaid">
+//  graph TD;
+//    A-- > B;
+//    A-- > C;
+//    B-- > D;
+//    C-- > D;
+// </pre>
+//
+
+import Flash from './flash';
+
+$.fn.renderMermaid = function renderMermaid() {
+  if (this.length === 0) return;
+
+  import(/* webpackChunkName: 'mermaid' */ 'blackst0ne-mermaid').then((mermaid) => {
+    mermaid.initialize({
+      loadOnStart: false,
+      theme: 'neutral',
+    });
+
+    mermaid.init(undefined, this);
+  }).catch((err) => {
+    Flash(`Can't load mermaid module: ${err}`);
+  });
+};
diff --git a/changelogs/unreleased/feature_add_mermaid.yml b/changelogs/unreleased/feature_add_mermaid.yml
new file mode 100644
index 0000000000000000000000000000000000000000..caeb5d3470dde777811b3a42d4297419414f9331
--- /dev/null
+++ b/changelogs/unreleased/feature_add_mermaid.yml
@@ -0,0 +1,5 @@
+---
+title: 'Add support of Mermaid (generation of diagrams and flowcharts from text)'
+merge_request: 15107
+author: Vitaliy @blackst0ne Klachkov
+type: added
diff --git a/config/initializers/math_lexer.rb b/config/initializers/math_lexer.rb
deleted file mode 100644
index 8a3388a267e83e11abceed020018131ce80e4cef..0000000000000000000000000000000000000000
--- a/config/initializers/math_lexer.rb
+++ /dev/null
@@ -1,2 +0,0 @@
-# Touch the lexers so it is registered with Rouge
-Rouge::Lexers::Math
diff --git a/config/initializers/plantuml_lexer.rb b/config/initializers/plantuml_lexer.rb
deleted file mode 100644
index e8a77b146fa0dc82ac4cd53348d7ae62cdac4216..0000000000000000000000000000000000000000
--- a/config/initializers/plantuml_lexer.rb
+++ /dev/null
@@ -1,2 +0,0 @@
-# Touch the lexers so it is registered with Rouge
-Rouge::Lexers::Plantuml
diff --git a/doc/user/markdown.md b/doc/user/markdown.md
index 454988b9b80cd989aa404facd4307d2710d50b0a..fb61e3609964fc0b056734b1d23e4fc246e1adaa 100644
--- a/doc/user/markdown.md
+++ b/doc/user/markdown.md
@@ -368,6 +368,37 @@ _Be advised that KaTeX only supports a [subset][katex-subset] of LaTeX._
 >**Note:**
 This also works for the asciidoctor `:stem: latexmath`. For details see the [asciidoctor user manual][asciidoctor-manual].
 
+### Mermaid
+
+> If this is not rendered correctly, see
+https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md#mermaid
+
+It is possible to generate diagrams and flowcharts from text using [Mermaid][mermaid]. 
+
+In order to generate a diagram or flowchart, you should write your text inside the `mermaid` block. 
+
+Example: 
+
+    ```mermaid
+    graph TD;
+      A-->B;
+      A-->C;
+      B-->D;
+      C-->D;
+    ```   
+
+Becomes:
+
+```mermaid
+graph TD;
+  A-->B;
+  A-->C;
+  B-->D;
+  C-->D;
+```   
+
+For details see the [Mermaid official page][mermaid].
+
 ## Standard Markdown
 
 ### Headers
@@ -814,6 +845,7 @@ A link starting with a `/` is relative to the wiki root.
 [^2]: This is my awesome footnote.
 
 [markdown.md]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md
+[mermaid]: https://mermaidjs.github.io/ "Mermaid website"
 [rouge]: http://rouge.jneen.net/ "Rouge website"
 [redcarpet]: https://github.com/vmg/redcarpet "Redcarpet website"
 [katex]: https://github.com/Khan/KaTeX "KaTeX website"
diff --git a/lib/banzai/filter/mermaid_filter.rb b/lib/banzai/filter/mermaid_filter.rb
new file mode 100644
index 0000000000000000000000000000000000000000..b545b947a2c7db864f6329441955f45440d7f908
--- /dev/null
+++ b/lib/banzai/filter/mermaid_filter.rb
@@ -0,0 +1,20 @@
+module Banzai
+  module Filter
+    class MermaidFilter < HTML::Pipeline::Filter
+      def call
+        doc.css('pre[lang="mermaid"]').add_class('mermaid')
+        doc.css('pre[lang="mermaid"]').add_class('js-render-mermaid')
+
+        # The `<code></code>` blocks are added in the lib/banzai/filter/syntax_highlight_filter.rb
+        # We want to keep context and consistency, so we the blocks are added for all filters.
+        # Details: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15107/diffs?diff_id=7962900#note_45495859
+        doc.css('pre[lang="mermaid"]').each do |pre|
+          document = pre.at('code')
+          document.replace(document.content)
+        end
+
+        doc
+      end
+    end
+  end
+end
diff --git a/lib/banzai/filter/syntax_highlight_filter.rb b/lib/banzai/filter/syntax_highlight_filter.rb
index 7da565043d114b857a29e31562ca564f60d689a3..a79a01548462856c68034991a8770cdc4b0c9138 100644
--- a/lib/banzai/filter/syntax_highlight_filter.rb
+++ b/lib/banzai/filter/syntax_highlight_filter.rb
@@ -14,23 +14,26 @@ module Banzai
       end
 
       def highlight_node(node)
-        language = node.attr('lang')
         code = node.text
-        css_classes = "code highlight"
-        lexer = lexer_for(language)
-        lang = lexer.tag
-
-        begin
-          code = Rouge::Formatters::HTMLGitlab.format(lex(lexer, code), tag: lang)
-
-          css_classes << " js-syntax-highlight #{lang}"
-        rescue
-          lang = nil
-          # Gracefully handle syntax highlighter bugs/errors to ensure
-          # users can still access an issue/comment/etc.
+        css_classes = 'code highlight js-syntax-highlight'
+        language = node.attr('lang')
+
+        if use_rouge?(language)
+          lexer = lexer_for(language)
+          language = lexer.tag
+
+          begin
+            code = Rouge::Formatters::HTMLGitlab.format(lex(lexer, code), tag: language)
+            css_classes << " #{language}"
+          rescue
+            # Gracefully handle syntax highlighter bugs/errors to ensure
+            # users can still access an issue/comment/etc.
+
+            language = nil
+          end
         end
 
-        highlighted = %(<pre class="#{css_classes}" lang="#{lang}" v-pre="true"><code>#{code}</code></pre>)
+        highlighted = %(<pre class="#{css_classes}" lang="#{language}" v-pre="true"><code>#{code}</code></pre>)
 
         # Extracted to a method to measure it
         replace_parent_pre_element(node, highlighted)
@@ -51,6 +54,10 @@ module Banzai
         # Replace the parent `pre` element with the entire highlighted block
         node.parent.replace(highlighted)
       end
+
+      def use_rouge?(language)
+        %w(math mermaid plantuml).exclude?(language)
+      end
     end
   end
 end
diff --git a/lib/banzai/pipeline/gfm_pipeline.rb b/lib/banzai/pipeline/gfm_pipeline.rb
index 3208abfc5381cdb1bf9d7a3aec4fb2a0b8b0d3ed..55874ad50a3645ad3665e3b0ce0ebbad2904089b 100644
--- a/lib/banzai/pipeline/gfm_pipeline.rb
+++ b/lib/banzai/pipeline/gfm_pipeline.rb
@@ -14,6 +14,7 @@ module Banzai
           Filter::SyntaxHighlightFilter,
 
           Filter::MathFilter,
+          Filter::MermaidFilter,
           Filter::UploadLinkFilter,
           Filter::VideoLinkFilter,
           Filter::ImageLazyLoadFilter,
diff --git a/lib/rouge/lexers/math.rb b/lib/rouge/lexers/math.rb
deleted file mode 100644
index 939b23a34212cb5dc7d70b55a86630656ba24f38..0000000000000000000000000000000000000000
--- a/lib/rouge/lexers/math.rb
+++ /dev/null
@@ -1,9 +0,0 @@
-module Rouge
-  module Lexers
-    class Math < PlainText
-      title "A passthrough lexer used for LaTeX input"
-      desc "PLEASE REFACTOR - this should be handled by SyntaxHighlightFilter"
-      tag 'math'
-    end
-  end
-end
diff --git a/lib/rouge/lexers/plantuml.rb b/lib/rouge/lexers/plantuml.rb
deleted file mode 100644
index 63c461764fc617e78eff150cc5e6035a72aa5bf9..0000000000000000000000000000000000000000
--- a/lib/rouge/lexers/plantuml.rb
+++ /dev/null
@@ -1,9 +0,0 @@
-module Rouge
-  module Lexers
-    class Plantuml < PlainText
-      title "A passthrough lexer used for PlantUML input"
-      desc "PLEASE REFACTOR - this should be handled by SyntaxHighlightFilter"
-      tag 'plantuml'
-    end
-  end
-end
diff --git a/package.json b/package.json
index cbcce40ffb95a228e50a2387f773ca20e5f6dbd0..591238dff73022a962c3d1ce882963a3578f9a72 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,7 @@
     "babel-plugin-transform-define": "^1.2.0",
     "babel-preset-latest": "^6.24.0",
     "babel-preset-stage-2": "^6.22.0",
+    "blackst0ne-mermaid": "^7.1.0-fixed",
     "bootstrap-sass": "^3.3.6",
     "brace-expansion": "^1.1.8",
     "compression-webpack-plugin": "^1.0.0",
diff --git a/spec/features/markdown_spec.rb b/spec/features/markdown_spec.rb
index b70d3060f050f4ee964e0c9a4745497f1f130b3d..cc1b187ff54ebc7664e025e2c2a9a33b5123a1fa 100644
--- a/spec/features/markdown_spec.rb
+++ b/spec/features/markdown_spec.rb
@@ -69,6 +69,12 @@ describe 'GitLab Markdown' do
         end
       end
 
+      it 'parses mermaid code block' do
+        aggregate_failures do
+          expect(doc).to have_selector('pre.code.js-render-mermaid')
+        end
+      end
+
       it 'parses strikethroughs' do
         expect(doc).to have_selector(%{del:contains("and this text doesn't")})
       end
diff --git a/spec/fixtures/markdown.md.erb b/spec/fixtures/markdown.md.erb
index 4f46e40ce7a4822c865e79164e7ca8f7e9f1809a..638cd8b07c82770beb877028c33668341aea8375 100644
--- a/spec/fixtures/markdown.md.erb
+++ b/spec/fixtures/markdown.md.erb
@@ -268,3 +268,37 @@ However the wrapping tags can not be mixed as such -
 ### Videos
 
 ![My Video](/assets/videos/gitlab-demo.mp4)
+
+### Mermaid
+
+> If this is not rendered correctly, see
+https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/user/markdown.md#mermaid
+
+It is possible to generate diagrams and flowcharts from text using [Mermaid][mermaid].
+
+In order to generate a diagram or flowchart, you should write your text inside the `mermaid` block.
+
+Example:
+
+    ```mermaid
+    graph TD;
+      A-->B;
+      A-->C;
+      B-->D;
+      C-->D;
+    ```
+
+Becomes:
+
+```mermaid
+graph TD;
+  A-->B;
+  A-->C;
+  B-->D;
+  C-->D;
+```
+
+For details see the [Mermaid official page][mermaid].
+
+[mermaid]: https://mermaidjs.github.io/ "Mermaid website"
+
diff --git a/spec/javascripts/notes_spec.js b/spec/javascripts/notes_spec.js
index 928a4b461cc2ba70b5cdb37d62eb98c7416a1e7e..63abac222c40bbcc21ddc4fd8d44c3d6646b43e5 100644
--- a/spec/javascripts/notes_spec.js
+++ b/spec/javascripts/notes_spec.js
@@ -4,8 +4,9 @@
 import 'autosize';
 import '~/gl_form';
 import '~/lib/utils/text_utility';
-import '~/render_gfm';
 import '~/render_math';
+import '~/render_mermaid';
+import '~/render_gfm';
 import '~/notes';
 
 (function() {
diff --git a/spec/lib/banzai/filter/mermaid_filter_spec.rb b/spec/lib/banzai/filter/mermaid_filter_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..532d25e121d4ccdad8375d3f13d11c9e110d3315
--- /dev/null
+++ b/spec/lib/banzai/filter/mermaid_filter_spec.rb
@@ -0,0 +1,12 @@
+require 'spec_helper'
+
+describe Banzai::Filter::MermaidFilter do
+  include FilterSpecHelper
+
+  it 'adds `js-render-mermaid` class to the `pre` tag' do
+    doc = filter("<pre class='code highlight js-syntax-highlight mermaid' lang='mermaid' v-pre='true'><code>graph TD;\n  A--&gt;B;\n</code></pre>")
+    result = doc.xpath('descendant-or-self::pre').first
+
+    expect(result[:class]).to include('js-render-mermaid')
+  end
+end
diff --git a/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb b/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
index 5a23e0e70cc3e8bd93207f8f93fb7d723821037c..9f2efa05a01d72117da138e19779db88e902f48f 100644
--- a/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
+++ b/spec/lib/banzai/filter/syntax_highlight_filter_spec.rb
@@ -31,7 +31,7 @@ describe Banzai::Filter::SyntaxHighlightFilter do
 
     it "highlights as plaintext" do
       result = filter('<pre><code lang="ruby">This is a test</code></pre>')
-      expect(result.to_html).to eq('<pre class="code highlight" lang="" v-pre="true"><code>This is a test</code></pre>')
+      expect(result.to_html).to eq('<pre class="code highlight js-syntax-highlight" lang="" v-pre="true"><code>This is a test</code></pre>')
     end
   end
 end
diff --git a/yarn.lock b/yarn.lock
index 1271c8a1ee39b791c0e39644bd44e317bf108fb3..1d6242d800b45e6bc1881bd0896d84f099502ed0 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -922,6 +922,17 @@ binary-extensions@^1.0.0:
   version "1.10.0"
   resolved "https://registry.yarnpkg.com/binary-extensions/-/binary-extensions-1.10.0.tgz#9aeb9a6c5e88638aad171e167f5900abe24835d0"
 
+blackst0ne-mermaid@^7.1.0-fixed:
+  version "7.1.0-fixed"
+  resolved "https://registry.yarnpkg.com/blackst0ne-mermaid/-/blackst0ne-mermaid-7.1.0-fixed.tgz#3707b3a113d78610e3068e18a588f46b4688de49"
+  dependencies:
+    d3 "3.5.17"
+    dagre-d3-renderer "^0.4.24"
+    dagre-layout "^0.8.0"
+    he "^1.1.1"
+    lodash "^4.17.4"
+    moment "^2.18.1"
+
 blob@0.0.4:
   version "0.0.4"
   resolved "https://registry.yarnpkg.com/blob/-/blob-0.0.4.tgz#bcf13052ca54463f30f9fc7e95b9a47630a94921"
@@ -1638,6 +1649,10 @@ custom-event@~1.0.0:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/custom-event/-/custom-event-1.0.1.tgz#5d02a46850adf1b4a317946a3928fccb5bfd0425"
 
+d3@3.5.17:
+  version "3.5.17"
+  resolved "https://registry.yarnpkg.com/d3/-/d3-3.5.17.tgz#bc46748004378b21a360c9fc7cf5231790762fb8"
+
 d3@^3.5.11:
   version "3.5.11"
   resolved "https://registry.yarnpkg.com/d3/-/d3-3.5.11.tgz#d130750eed0554db70e8432102f920a12407b69c"
@@ -1654,6 +1669,22 @@ d@^0.1.1:
   dependencies:
     es5-ext "~0.10.2"
 
+dagre-d3-renderer@^0.4.24:
+  version "0.4.24"
+  resolved "https://registry.yarnpkg.com/dagre-d3-renderer/-/dagre-d3-renderer-0.4.24.tgz#b36ce2fe4ea20de43e7698627c6ede2a9f15ec45"
+  dependencies:
+    d3 "3.5.17"
+    dagre-layout "^0.8.0"
+    graphlib "^2.1.1"
+    lodash "^4.17.4"
+
+dagre-layout@^0.8.0:
+  version "0.8.0"
+  resolved "https://registry.yarnpkg.com/dagre-layout/-/dagre-layout-0.8.0.tgz#7147b6afb655602f855158dfea171db9aa98d4ff"
+  dependencies:
+    graphlib "^2.1.1"
+    lodash "^4.17.4"
+
 dashdash@^1.12.0:
   version "1.14.1"
   resolved "https://registry.yarnpkg.com/dashdash/-/dashdash-1.14.1.tgz#853cfa0f7cbe2fed5de20326b8dd581035f6e2f0"
@@ -2852,6 +2883,12 @@ graceful-fs@^4.1.11, graceful-fs@^4.1.2, graceful-fs@^4.1.6, graceful-fs@^4.1.9:
   version "1.0.1"
   resolved "https://registry.yarnpkg.com/graceful-readlink/-/graceful-readlink-1.0.1.tgz#4cafad76bc62f02fa039b2f94e9a3dd3a391a725"
 
+graphlib@^2.1.1:
+  version "2.1.1"
+  resolved "https://registry.yarnpkg.com/graphlib/-/graphlib-2.1.1.tgz#42352c52ba2f4d035cb566eb91f7395f76ebc951"
+  dependencies:
+    lodash "^4.11.1"
+
 gzip-size@3.0.0, gzip-size@^3.0.0:
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/gzip-size/-/gzip-size-3.0.0.tgz#546188e9bdc337f673772f81660464b389dce520"
@@ -2946,7 +2983,7 @@ hawk@~3.1.3:
     hoek "2.x.x"
     sntp "1.x.x"
 
-he@^1.1.0:
+he@^1.1.0, he@^1.1.1:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/he/-/he-1.1.1.tgz#93410fd21b009735151f8868c2f271f3427e23fd"
 
@@ -3942,7 +3979,7 @@ lodash@^3.8.0:
   version "3.10.1"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-3.10.1.tgz#5bf45e8e49ba4189e17d482789dfd15bd140b7b6"
 
-lodash@^4.0.0, lodash@^4.14.0, lodash@^4.17.2, lodash@^4.17.4, lodash@^4.2.0, lodash@^4.3.0, lodash@^4.5.0:
+lodash@^4.0.0, lodash@^4.11.1, lodash@^4.14.0, lodash@^4.17.2, lodash@^4.17.4, lodash@^4.2.0, lodash@^4.3.0, lodash@^4.5.0:
   version "4.17.4"
   resolved "https://registry.yarnpkg.com/lodash/-/lodash-4.17.4.tgz#78203a4d1c328ae1d86dca6460e369b57f4055ae"
 
@@ -4156,6 +4193,10 @@ moment@2.x:
   version "2.17.1"
   resolved "https://registry.yarnpkg.com/moment/-/moment-2.17.1.tgz#fed9506063f36b10f066c8b59a144d7faebe1d82"
 
+moment@^2.18.1:
+  version "2.19.2"
+  resolved "https://registry.yarnpkg.com/moment/-/moment-2.19.2.tgz#8a7f774c95a64550b4c7ebd496683908f9419dbe"
+
 monaco-editor@0.10.0:
   version "0.10.0"
   resolved "https://registry.yarnpkg.com/monaco-editor/-/monaco-editor-0.10.0.tgz#6604932585fe9c1f993f000a503d0d20fbe5896a"