diff --git a/activesupport/lib/active_support/duration/iso8601_serializer.rb b/activesupport/lib/active_support/duration/iso8601_serializer.rb
index 05c6a083a9f710dbff91b46141400b685b20e996..eb8136e2082a7526e395afbbbe34079ca17218be 100644
--- a/activesupport/lib/active_support/duration/iso8601_serializer.rb
+++ b/activesupport/lib/active_support/duration/iso8601_serializer.rb
@@ -12,8 +12,10 @@ def initialize(duration, precision: nil)
 
       # Builds and returns output string.
       def serialize
-        output = 'P'
         parts, sign = normalize
+        return "PT0S".freeze if parts.empty?
+
+        output = 'P'
         output << "#{parts[:years]}Y"   if parts.key?(:years)
         output << "#{parts[:months]}M"  if parts.key?(:months)
         output << "#{parts[:weeks]}W"   if parts.key?(:weeks)
diff --git a/activesupport/test/core_ext/duration_test.rb b/activesupport/test/core_ext/duration_test.rb
index 502e2811fa3effc1d263ed8516fb60a03c7312e8..a24915dfef68b262e9704ae5d1ad66a485ae5b0e 100644
--- a/activesupport/test/core_ext/duration_test.rb
+++ b/activesupport/test/core_ext/duration_test.rb
@@ -279,6 +279,7 @@ def test_iso8601_output
       ['PT1S',          1.second                         ],
       ['PT1.4S',        (1.4).seconds                    ],
       ['P1Y1M1DT1H',    1.year + 1.month + 1.day + 1.hour],
+      ['PT0S',          0.minutes                        ],
     ]
     expectations.each do |expected_output, duration|
       assert_equal expected_output, duration.iso8601, expected_output.inspect