diff --git a/changelog/fix_an_incorrect_autocorrect_for_rails_validation.md b/changelog/fix_an_incorrect_autocorrect_for_rails_validation.md
new file mode 100644
index 0000000000..e6455ad3ca
--- /dev/null
+++ b/changelog/fix_an_incorrect_autocorrect_for_rails_validation.md
@@ -0,0 +1 @@
+* [#1270](https://github.com/rubocop/rubocop-rails/issues/1270): Fix an incorrect autocorrect for `Rails/Validation` when using `validates_size_of`. ([@koic][])
diff --git a/lib/rubocop/cop/rails/validation.rb b/lib/rubocop/cop/rails/validation.rb
index 534af1b260..5499f3991d 100644
--- a/lib/rubocop/cop/rails/validation.rb
+++ b/lib/rubocop/cop/rails/validation.rb
@@ -29,7 +29,7 @@ module Rails
       #   validates :foo, numericality: true
       #   validates :foo, presence: true
       #   validates :foo, absence: true
-      #   validates :foo, size: true
+      #   validates :foo, length: true
       #   validates :foo, uniqueness: true
       #
       class Validation < Base
@@ -120,7 +120,9 @@ def correct_validate_type_for_array(corrector, node, arguments, loc)
         end
 
         def validate_type(node)
-          node.method_name.to_s.split('_')[1]
+          type = node.method_name.to_s.split('_')[1]
+
+          type == 'size' ? 'length' : type
         end
 
         def frozen_array_argument?(argument)
diff --git a/spec/rubocop/cop/rails/validation_spec.rb b/spec/rubocop/cop/rails/validation_spec.rb
index 0254dabf4e..e789842d6d 100644
--- a/spec/rubocop/cop/rails/validation_spec.rb
+++ b/spec/rubocop/cop/rails/validation_spec.rb
@@ -28,6 +28,8 @@
     described_class::TYPES.each do |type|
       context "with validates_#{type}_of" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates :full_name, :birth_date, #{type}: true"
         end
 
@@ -40,6 +42,8 @@
 
       context "with validates_#{type}_of when method arguments are enclosed in parentheses" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates(:full_name, :birth_date, #{type}: true)"
         end
 
@@ -52,6 +56,8 @@
 
       context "with validates_#{type}_of when attributes are specified with array literal" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates :full_name, :birth_date, #{type}: true"
         end
 
@@ -64,6 +70,8 @@
 
       context "with validates_#{type}_of when attributes are specified with frozen array literal" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates :full_name, :birth_date, #{type}: true"
         end
 
@@ -76,6 +84,8 @@
 
       context "with validates_#{type}_of when attributes are specified with symbol array literal" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates :full_name, :birth_date, #{type}: true"
         end
 
@@ -88,6 +98,8 @@
 
       context "with validates_#{type}_of when attributes are specified with frozen symbol array literal" do
         let(:autocorrected_source) do
+          type = 'length' if type == 'size'
+
           "validates :full_name, :birth_date, #{type}: true"
         end