diff --git a/changelog/fix_error_message_for_dept.md b/changelog/fix_error_message_for_dept.md
new file mode 100644
index 00000000000..8c50df131a8
--- /dev/null
+++ b/changelog/fix_error_message_for_dept.md
@@ -0,0 +1 @@
+* [#9752](https://github.com/rubocop/rubocop/issues/9752): Improve error message for top level department used in configuration. ([@jonas054][])
diff --git a/lib/rubocop/config_validator.rb b/lib/rubocop/config_validator.rb
index f8c7d3ecd1a..87656273736 100644
--- a/lib/rubocop/config_validator.rb
+++ b/lib/rubocop/config_validator.rb
@@ -104,12 +104,9 @@ def alert_about_unrecognized_cops(invalid_cop_names)
         # to do so than to pass the value around to various methods.
         next if name == 'inherit_mode'
 
-        suggestions = NameSimilarity.find_similar_names(name, Cop::Registry.global.map(&:cop_name))
-        suggestion = "Did you mean `#{suggestions.join('`, `')}`?" if suggestions.any?
-
         message = <<~MESSAGE.rstrip
-          unrecognized cop #{name} found in #{smart_loaded_path}
-          #{suggestion}
+          unrecognized cop or department #{name} found in #{smart_loaded_path}
+          #{suggestion(name)}
         MESSAGE
 
         unknown_cops << message
@@ -117,6 +114,22 @@ def alert_about_unrecognized_cops(invalid_cop_names)
       raise ValidationError, unknown_cops.join("\n") if unknown_cops.any?
     end
 
+    def suggestion(name)
+      registry = Cop::Registry.global
+      departments = registry.departments.map(&:to_s)
+      suggestions = NameSimilarity.find_similar_names(name, departments + registry.map(&:cop_name))
+      if suggestions.any?
+        "Did you mean `#{suggestions.join('`, `')}`?"
+      else
+        # Department names can contain slashes, e.g. Chef/Correctness, but there's no support for
+        # the concept of higher level departments in RuboCop. It's a flat structure. So if the user
+        # tries to configure a "top level department", we hint that it's the bottom level
+        # departments that should be configured.
+        suggestions = departments.select { |department| department.start_with?("#{name}/") }
+        "#{name} is not a department. Use `#{suggestions.join('`, `')}`." if suggestions.any?
+      end
+    end
+
     def validate_syntax_cop
       syntax_config = @config['Lint/Syntax']
       default_config = ConfigLoader.default_configuration['Lint/Syntax']
diff --git a/spec/rubocop/cli_spec.rb b/spec/rubocop/cli_spec.rb
index c84bc7c5cb4..75a4e43bfd9 100644
--- a/spec/rubocop/cli_spec.rb
+++ b/spec/rubocop/cli_spec.rb
@@ -1380,6 +1380,8 @@ def meow_at_4am?
         Layout/LyneLenth:
           Enabled: true
           Max: 100
+        Linth:
+          Enabled: false
         Lint/LiteralInCondition:
           Enabled: true
         Style/AlignHash:
@@ -1389,11 +1391,13 @@ def meow_at_4am?
       expect(cli.run(%w[--format simple example])).to eq(2)
       expect($stderr.string)
         .to eq(<<~OUTPUT)
-          Error: unrecognized cop Layout/LyneLenth found in example/.rubocop.yml
+          Error: unrecognized cop or department Layout/LyneLenth found in example/.rubocop.yml
           Did you mean `Layout/LineLength`?
-          unrecognized cop Lint/LiteralInCondition found in example/.rubocop.yml
+          unrecognized cop or department Linth found in example/.rubocop.yml
+          Did you mean `Lint`?
+          unrecognized cop or department Lint/LiteralInCondition found in example/.rubocop.yml
           Did you mean `Lint/LiteralAsCondition`?
-          unrecognized cop Style/AlignHash found in example/.rubocop.yml
+          unrecognized cop or department Style/AlignHash found in example/.rubocop.yml
           Did you mean `Style/Alias`, `Style/OptionHash`?
         OUTPUT
     end
@@ -1688,7 +1692,7 @@ def method(foo, bar, qux, fred, arg5, f) end #{'#' * 85}
         YAML
         expect(cli.run(['example1.rb'])).to eq(2)
         expect($stderr.string.strip).to eq(
-          'Error: unrecognized cop Syntax/Whatever found in .rubocop.yml'
+          'Error: unrecognized cop or department Syntax/Whatever found in .rubocop.yml'
         )
       end
     end
diff --git a/spec/rubocop/config_loader_spec.rb b/spec/rubocop/config_loader_spec.rb
index a9af310322b..9f363d134f7 100644
--- a/spec/rubocop/config_loader_spec.rb
+++ b/spec/rubocop/config_loader_spec.rb
@@ -540,11 +540,12 @@
     context 'when a department is disabled', :restore_registry do
       let(:file_path) { '.rubocop.yml' }
 
-      shared_examples 'resolves enabled/disabled for all cops' do |enabled_by_default, disabled_by_default|
+      shared_examples 'resolves enabled/disabled for all ' \
+                      'cops' do |enabled_by_default, disabled_by_default, custom_dept_to_disable|
         before { stub_cop_class('RuboCop::Cop::Foo::Bar::Baz') }
 
         it "handles EnabledByDefault: #{enabled_by_default}, " \
-           "DisabledByDefault: #{disabled_by_default}" do
+           "DisabledByDefault: #{disabled_by_default} with disabled #{custom_dept_to_disable}" do
           create_file('grandparent_rubocop.yml', <<~YAML)
             Naming/FileName:
               Enabled: pending
@@ -573,7 +574,7 @@
             Naming:
               Enabled: false
 
-            Foo/Bar:
+            #{custom_dept_to_disable}:
               Enabled: false
           YAML
           create_file(file_path, <<~YAML)
@@ -603,6 +604,15 @@ def enabled?(cop)
             configuration_from_file.for_cop(cop)['Enabled']
           end
 
+          if custom_dept_to_disable == 'Foo'
+            message = <<~'OUTPUT'.chomp
+              unrecognized cop or department Foo found in parent_rubocop.yml
+              Foo is not a department. Use `Foo/Bar`.
+            OUTPUT
+            expect { enabled?('Foo/Bar/Baz') }.to raise_error(RuboCop::ValidationError, message)
+            next
+          end
+
           # Department disabled in parent config, cop enabled in child.
           expect(enabled?('Metrics/MethodLength')).to be(true)
 
@@ -641,9 +651,10 @@ def enabled?(cop)
         end
       end
 
-      include_examples 'resolves enabled/disabled for all cops', false, false
-      include_examples 'resolves enabled/disabled for all cops', false, true
-      include_examples 'resolves enabled/disabled for all cops', true, false
+      include_examples 'resolves enabled/disabled for all cops', false, false, 'Foo/Bar'
+      include_examples 'resolves enabled/disabled for all cops', false, true, 'Foo/Bar'
+      include_examples 'resolves enabled/disabled for all cops', true, false, 'Foo/Bar'
+      include_examples 'resolves enabled/disabled for all cops', false, false, 'Foo'
     end
 
     context 'when a third party require defines a new gem', :restore_registry do
diff --git a/spec/rubocop/config_spec.rb b/spec/rubocop/config_spec.rb
index c01b0ee886d..f17a08d77ea 100644
--- a/spec/rubocop/config_spec.rb
+++ b/spec/rubocop/config_spec.rb
@@ -31,7 +31,7 @@
       it 'raises an validation error' do
         expect { configuration }.to raise_error(
           RuboCop::ValidationError,
-          'unrecognized cop LyneLenth found in .rubocop.yml'
+          'unrecognized cop or department LyneLenth found in .rubocop.yml'
         )
       end
     end