diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index a5fe6d65d9c122ffe147067e849a950773b594d5..74d57180fe935727553fc735545b90fbfbc35c29 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Corrected Inflector#underscore handling of multiple successive acroynms.
+
+    *James Le Cuirot*
+
 *   Delegation now works with ruby reserved words passed to `:to` option.
 
     Fixes #16956.
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index f35e71ce8115950e912b6ed856688c07aaa1c3d1..5b75dd65d036027cbeed78d2e144f4bfbf44208b 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -91,7 +91,7 @@ def camelize(term, uppercase_first_letter = true)
     def underscore(camel_cased_word)
       return camel_cased_word unless camel_cased_word =~ /[A-Z-]|::/
       word = camel_cased_word.to_s.gsub('::', '/')
-      word.gsub!(/(?:([A-Za-z\d])|\b)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1}#{$1 && '_'}#{$2.downcase}" }
+      word.gsub!(/(?<=([A-Za-z\d])|\b)(#{inflections.acronym_regex})(?=\b|[^a-z])/) { "#{$1 && '_'}#{$2.downcase}" }
       word.gsub!(/([A-Z\d]+)([A-Z][a-z])/,'\1_\2')
       word.gsub!(/([a-z\d])([A-Z])/,'\1_\2')
       word.tr!("-", "_")
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index b37f31bc5f0a5450320dc27ebb01b63e04c17446..5446c5ec3c5aa474cba41cab2cbcaba381cdb8c8 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -125,6 +125,9 @@ def test_acronyms
       ["PhDRequired",       "phd_required",       "PhD required",       "PhD Required"],
       ["IRoRU",             "i_ror_u",            "I RoR u",            "I RoR U"],
       ["RESTfulHTTPAPI",    "restful_http_api",   "RESTful HTTP API",   "RESTful HTTP API"],
+      ["HTTP::RESTful",     "http/restful",       "HTTP/RESTful",       "HTTP/RESTful"],
+      ["HTTP::RESTfulAPI",  "http/restful_api",   "HTTP/RESTful API",   "HTTP/RESTful API"],
+      ["APIRESTful",        "api_restful",        "API RESTful",        "API RESTful"],
 
       # misdirection
       ["Capistrano",        "capistrano",         "Capistrano",       "Capistrano"],