diff --git a/lib/irb/nesting_parser.rb b/lib/irb/nesting_parser.rb
index 23eeeaf6cf..5aa940cc28 100644
--- a/lib/irb/nesting_parser.rb
+++ b/lib/irb/nesting_parser.rb
@@ -12,6 +12,8 @@ module IRB
         skip = false
         last_tok, state, args = opens.last
         case state
+        when :in_alias_undef
+          skip = t.event == :on_kw
         when :in_unquoted_symbol
           unless IGNORE_TOKENS.include?(t.event)
             opens.pop
@@ -130,6 +132,10 @@ module IRB
                 opens.pop
                 opens << [t, nil]
               end
+            when 'alias'
+              opens << [t, :in_alias_undef, 2]
+            when 'undef'
+              opens << [t, :in_alias_undef, 1]
             when 'elsif', 'else', 'when'
               opens.pop
               opens << [t, nil]
@@ -174,6 +180,10 @@ module IRB
           pending_heredocs.reverse_each { |t| opens << [t, nil] }
           pending_heredocs = []
         end
+        if opens.last && opens.last[1] == :in_alias_undef && !IGNORE_TOKENS.include?(t.event) && t.event != :on_heredoc_end
+          tok, state, arg = opens.pop
+          opens << [tok, state, arg - 1] if arg >= 1
+        end
         yield t, opens if block_given?
       end
       opens.map(&:first) + pending_heredocs.reverse
diff --git a/lib/irb/ruby-lex.rb b/lib/irb/ruby-lex.rb
index 4bce2aa6b2..754acfad05 100644
--- a/lib/irb/ruby-lex.rb
+++ b/lib/irb/ruby-lex.rb
@@ -290,7 +290,7 @@ module IRB
         when :on_embdoc_beg
           indent_level = 0
         else
-          indent_level += 1
+          indent_level += 1 unless t.tok == 'alias' || t.tok == 'undef'
         end
       end
       indent_level
diff --git a/test/irb/test_nesting_parser.rb b/test/irb/test_nesting_parser.rb
index ea3a23aaf5..2482d40081 100644
--- a/test/irb/test_nesting_parser.rb
+++ b/test/irb/test_nesting_parser.rb
@@ -280,6 +280,44 @@ module TestIRB
       end
     end
 
+    def test_undef_alias
+      codes = [
+        'undef foo',
+        'alias foo bar',
+        'undef !',
+        'alias + -',
+        'alias $a $b',
+        'undef do',
+        'alias do do',
+        'undef :do',
+        'alias :do :do',
+        'undef :"#{alias do do}"',
+        'alias :"#{undef do}" do',
+        'alias do :"#{undef do}"'
+      ]
+      code_with_comment = <<~EOS
+        undef #
+        #
+        do #
+        alias #
+        #
+        do #
+        #
+        do #
+      EOS
+      code_with_heredoc = <<~EOS
+        <<~A; alias
+        A
+        :"#{<<~A}"
+        A
+        do
+      EOS
+      [*codes, code_with_comment, code_with_heredoc].each do |code|
+        opens = IRB::NestingParser.open_tokens(IRB::RubyLex.ripper_lex_without_warning('(' + code + "\nif"))
+        assert_equal(%w[( if], opens.map(&:tok))
+      end
+    end
+
     def test_case_in
       if Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.7.0')
         pend 'This test requires ruby version that supports case-in syntax'