diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index a2c872cb89578..ba48358879e11 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   `ActiveSupport::Inflector::Inflections#clear(:acronyms)` is now supported,
+    and `inflector.clear` / `inflector.clear(:all)` also clears acronyms.
+
+    *Alex Ghiculescu*, *Oliver Peate*
+
+
 ## Rails 7.0.0.alpha2 (September 15, 2021) ##
 
 *   No changes.
diff --git a/activesupport/lib/active_support/inflector/inflections.rb b/activesupport/lib/active_support/inflector/inflections.rb
index 85b00ee90b9d2..a9943a8838a41 100644
--- a/activesupport/lib/active_support/inflector/inflections.rb
+++ b/activesupport/lib/active_support/inflector/inflections.rb
@@ -222,15 +222,24 @@ def human(rule, replacement)
       # Clears the loaded inflections within a given scope (default is
       # <tt>:all</tt>). Give the scope as a symbol of the inflection type, the
       # options are: <tt>:plurals</tt>, <tt>:singulars</tt>, <tt>:uncountables</tt>,
-      # <tt>:humans</tt>.
+      # <tt>:humans</tt>, <tt>:acronyms</tt>.
       #
       #   clear :all
       #   clear :plurals
       def clear(scope = :all)
         case scope
         when :all
-          @plurals, @singulars, @uncountables, @humans = [], [], Uncountables.new, []
-        else
+          clear(:acronyms)
+          clear(:plurals)
+          clear(:singulars)
+          clear(:uncountables)
+          clear(:humans)
+        when :acronyms
+          @acronyms = {}
+          define_acronym_regex_patterns
+        when :uncountables
+          @uncountables = Uncountables.new
+        when :plurals, :singulars, :humans
           instance_variable_set "@#{scope}", []
         end
       end
diff --git a/activesupport/test/inflector_test.rb b/activesupport/test/inflector_test.rb
index 5aac45a9481f9..7814326e0026f 100644
--- a/activesupport/test/inflector_test.rb
+++ b/activesupport/test/inflector_test.rb
@@ -469,6 +469,18 @@ def test_clear_#{inflection_type}
     RUBY
   end
 
+  def test_clear_acronyms_resets_to_reusable_state
+    ActiveSupport::Inflector.inflections.clear(:acronyms)
+
+    assert_empty ActiveSupport::Inflector.inflections.acronyms
+
+    ActiveSupport::Inflector.inflections do |inflect|
+      inflect.acronym "HTML"
+    end
+
+    assert_equal "HTML", "html".titleize
+  end
+
   def test_inflector_locality
     ActiveSupport::Inflector.inflections(:es) do |inflect|
       inflect.plural(/$/, "s")
@@ -512,6 +524,7 @@ def test_clear_all
       inflect.singular(/(database)s$/i, '\1')
       inflect.uncountable("series")
       inflect.human("col_rpted_bugs", "Reported bugs")
+      inflect.acronym("HTML")
 
       inflect.clear :all
 
@@ -519,6 +532,7 @@ def test_clear_all
       assert_empty inflect.singulars
       assert_empty inflect.uncountables
       assert_empty inflect.humans
+      assert_empty inflect.acronyms
     end
   end
 
@@ -529,6 +543,7 @@ def test_clear_with_default
       inflect.singular(/(database)s$/i, '\1')
       inflect.uncountable("series")
       inflect.human("col_rpted_bugs", "Reported bugs")
+      inflect.acronym("HTML")
 
       inflect.clear
 
@@ -536,6 +551,22 @@ def test_clear_with_default
       assert_empty inflect.singulars
       assert_empty inflect.uncountables
       assert_empty inflect.humans
+      assert_empty inflect.acronyms
+    end
+  end
+
+  def test_clear_all_resets_camelize_and_underscore_regexes
+    ActiveSupport::Inflector.inflections do |inflect|
+      # ensure any data is present
+      inflect.acronym("HTTP")
+      assert_equal "http_s", "HTTPS".underscore
+      assert_equal "Https", "https".camelize
+
+      inflect.clear :all
+
+      assert_empty inflect.acronyms
+      assert_equal "https", "HTTPS".underscore
+      assert_equal "Https", "https".camelize
     end
   end
 
@@ -592,7 +623,7 @@ def test_clear_with_default
     end
   end
 
-  %w(plurals singulars uncountables humans acronyms).each do |scope|
+  %i(plurals singulars uncountables humans).each do |scope|
     define_method("test_clear_inflections_with_#{scope}") do
       # clear the inflections
       ActiveSupport::Inflector.inflections do |inflect|
@@ -601,4 +632,11 @@ def test_clear_with_default
       end
     end
   end
+
+  def test_clear_inflections_with_acronyms
+    ActiveSupport::Inflector.inflections do |inflect|
+      inflect.clear(:acronyms)
+      assert_equal({}, inflect.acronyms)
+    end
+  end
 end