diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae5a7fcb0e91..82bcfea876c3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -28,6 +28,7 @@
 * Added ability to download shared rubocop config files from remote urls. ([@ptrippett][])
 * [#1601](https://github.com/bbatsov/rubocop/issues/1601): Add `IgnoreEmptyMethods` config parameter for `Lint/UnusedMethodArgument` and `IgnoreEmptyBlocks` config parameter for `Lint/UnusedBlockArgument` cops. ([@alexdowad][])
 * [#1729](https://github.com/bbatsov/rubocop/issues/1729): `Style/MethodDefParentheses` supports new 'require_no_parentheses_except_multiline' style. ([@alexdowad][])
+* [#2173](https://github.com/bbatsov/rubocop/issues/2173): `Style/AlignParameters` also checks parameter alignment for method definitions. ([@alexdowad][])
 
 ### Bug Fixes
 
diff --git a/lib/rubocop/cop/style/align_parameters.rb b/lib/rubocop/cop/style/align_parameters.rb
index d18cfcbf9ad2..b4924d4caa03 100644
--- a/lib/rubocop/cop/style/align_parameters.rb
+++ b/lib/rubocop/cop/style/align_parameters.rb
@@ -3,23 +3,33 @@
 module RuboCop
   module Cop
     module Style
-      # Here we check if the parameters on a multi-line method call are
-      # aligned.
+      # Here we check if the parameters on a multi-line method call or
+      # definition are aligned.
       class AlignParameters < Cop
         include AutocorrectAlignment
-
-        MSG = 'Align the parameters of a method call if they span ' \
-              'more than one line.'
+        include OnMethodDef
 
         def on_send(node)
           _receiver, method, *args = *node
 
           return if method == :[]=
-          return if args.size <= 1
+          return if args.size < 2
+
+          check_alignment(args, base_column(node, args))
+        end
 
+        def on_method_def(node, _method_name, args, _body)
+          args = args.children
+          return if args.size < 2
           check_alignment(args, base_column(node, args))
         end
 
+        def message(node)
+          type = node.parent.send_type? ? 'call' : 'definition'
+          "Align the parameters of a method #{type} if they span " \
+          'more than one line.'
+        end
+
         private
 
         def fixed_indentation?
@@ -38,7 +48,9 @@ def base_column(node, args)
         end
 
         def target_method_lineno(node)
-          if node.loc.selector
+          if node.def_type? || node.defs_type?
+            node.loc.keyword.line
+          elsif node.loc.selector
             node.loc.selector.line
           else
             # l.(1) has no selector, so we use the opening parenthesis instead
diff --git a/spec/rubocop/cop/style/align_parameters_spec.rb b/spec/rubocop/cop/style/align_parameters_spec.rb
index 9e60a0b6ef5f..5e3ef336e259 100644
--- a/spec/rubocop/cop/style/align_parameters_spec.rb
+++ b/spec/rubocop/cop/style/align_parameters_spec.rb
@@ -190,6 +190,94 @@
       expect(cop.offenses).to be_empty
     end
 
+    context 'method definitions' do
+      it 'registers an offense for parameters with single indent' do
+        inspect_source(cop, ['def method(a,',
+                             '  b)',
+                             'end'])
+        expect(cop.offenses.size).to eq 1
+        expect(cop.offenses.first.to_s).to match(/method definition/)
+      end
+
+      it 'registers an offense for parameters with double indent' do
+        inspect_source(cop, ['def method(a,',
+                             '    b)',
+                             'end'])
+        expect(cop.offenses.size).to eq 1
+      end
+
+      it 'accepts parameter lists on a single line' do
+        inspect_source(cop, ['def method(a, b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts proper indentation' do
+        inspect_source(cop, ['def method(a,',
+                             '           b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts the first parameter being on a new row' do
+        inspect_source(cop, ['def method(',
+                             '  a,',
+                             '  b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts a method definition without parameters' do
+        inspect_source(cop, ['def method',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it "doesn't get confused by splat" do
+        inspect_source(cop, ['def func2(a,',
+                             '         *b,',
+                             '          c)',
+                             'end'])
+        expect(cop.offenses.size).to eq 1
+        expect(cop.highlights).to eq(['*b'])
+      end
+
+      it 'auto-corrects alignment' do
+        new_source = autocorrect_source(cop, ['def method(a,',
+                                              '    b)',
+                                              'end'])
+        expect(new_source).to eq(['def method(a,',
+                                  '           b)',
+                                  'end'].join("\n"))
+      end
+
+      context 'defining self.method' do
+        it 'registers an offense for parameters with single indent' do
+          inspect_source(cop, ['def self.method(a,',
+                               '  b)',
+                               'end'])
+          expect(cop.offenses.size).to eq 1
+          expect(cop.offenses.first.to_s).to match(/method definition/)
+        end
+
+        it 'accepts proper indentation' do
+          inspect_source(cop, ['def self.method(a,',
+                               '                b)',
+                               'end'])
+          expect(cop.offenses).to be_empty
+        end
+
+        it 'auto-corrects alignment' do
+          new_source = autocorrect_source(cop, ['def self.method(a,',
+                                                '    b)',
+                                                'end'])
+          expect(new_source).to eq(['def self.method(a,',
+                                    '                b)',
+                                    'end'].join("\n"))
+        end
+      end
+    end
+
     context 'assigned methods' do
       it 'accepts the first parameter being on a new row' do
         inspect_source(cop, [' assigned_value = match(',
@@ -388,6 +476,94 @@
       end
     end
 
+    context 'method definitions' do
+      it 'registers an offense for parameters aligned to first param' do
+        inspect_source(cop, ['def method(a,',
+                             '           b)',
+                             'end'])
+        expect(cop.offenses.size).to eq 1
+        expect(cop.offenses.first.to_s).to match(/method definition/)
+      end
+
+      it 'registers an offense for parameters with double indent' do
+        inspect_source(cop, ['def method(a,',
+                             '    b)',
+                             'end'])
+        expect(cop.offenses.size).to eq 1
+      end
+
+      it 'accepts parameter lists on a single line' do
+        inspect_source(cop, ['def method(a, b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts proper indentation' do
+        inspect_source(cop, ['def method(a,',
+                             '  b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts the first parameter being on a new row' do
+        inspect_source(cop, ['def method(',
+                             '  a,',
+                             '  b)',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it 'accepts a method definition without parameters' do
+        inspect_source(cop, ['def method',
+                             'end'])
+        expect(cop.offenses).to be_empty
+      end
+
+      it "doesn't get confused by splat" do
+        inspect_source(cop, ['def func2(a,',
+                             '         *b,',
+                             '          c)',
+                             'end'])
+        expect(cop.offenses).not_to be_empty
+        expect(cop.highlights).to include '*b'
+      end
+
+      it 'auto-corrects alignment' do
+        new_source = autocorrect_source(cop, ['def method(a,',
+                                              '    b)',
+                                              'end'])
+        expect(new_source).to eq(['def method(a,',
+                                  '  b)',
+                                  'end'].join("\n"))
+      end
+
+      context 'defining self.method' do
+        it 'registers an offense for parameters aligned to first param' do
+          inspect_source(cop, ['def self.method(a,',
+                               '                b)',
+                               'end'])
+          expect(cop.offenses.size).to eq 1
+          expect(cop.offenses.first.to_s).to match(/method definition/)
+        end
+
+        it 'accepts proper indentation' do
+          inspect_source(cop, ['def self.method(a,',
+                               '  b)',
+                               'end'])
+          expect(cop.offenses).to be_empty
+        end
+
+        it 'auto-corrects alignment' do
+          new_source = autocorrect_source(cop, ['def self.method(a,',
+                                                '    b)',
+                                                'end'])
+          expect(new_source).to eq(['def self.method(a,',
+                                    '  b)',
+                                    'end'].join("\n"))
+        end
+      end
+    end
+
     context 'assigned methods' do
       context 'with IndentationWidth:Width set to 4' do
         let(:indentation_width) { 4 }