diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index d664bc2027..9c27c68919 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,14 @@
+*   Fix acronym support in `humanize`
+
+    Acronym inflections are stored with lowercase keys in the hash but
+    the match wasn't being lowercased before being looked up in the hash.
+    This shouldn't have any performance impact because before it would
+    fail to find the acronym and perform the `downcase` operation anyway.
+
+    Fixes #31052.
+
+    *Andrew White*
+
 *   Add same method signature for `Time#prev_year` and `Time#next_year`
     in accordance with `Date#prev_year`, `Date#next_year`.
 
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 72de42b1c3..3357b5eb32 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -138,7 +138,7 @@ module ActiveSupport
       result.tr!("_".freeze, " ".freeze)
 
       result.gsub!(/([a-z\d]*)/i) do |match|
-        "#{inflections.acronyms[match] || match.downcase}"
+        "#{inflections.acronyms[match.downcase] || match.downcase}"
       end
 
       if capitalize
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index eeec0ab1a5..6cc1039d8e 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -356,6 +356,19 @@ class InflectorTest < ActiveSupport::TestCase
     assert_equal("Col rpted bugs", ActiveSupport::Inflector.humanize("COL_rpted_bugs"))
   end
 
+  def test_humanize_with_acronyms
+    ActiveSupport::Inflector.inflections do |inflect|
+      inflect.acronym "LAX"
+      inflect.acronym "SFO"
+    end
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("LAX ROUNDTRIP TO SFO"))
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("LAX ROUNDTRIP TO SFO", capitalize: false))
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("lax roundtrip to sfo"))
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("lax roundtrip to sfo", capitalize: false))
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("Lax Roundtrip To Sfo"))
+    assert_equal("LAX roundtrip to SFO", ActiveSupport::Inflector.humanize("Lax Roundtrip To Sfo", capitalize: false))
+  end
+
   def test_constantize
     run_constantize_tests_on do |string|
       ActiveSupport::Inflector.constantize(string)