diff --git a/CHANGELOG.md b/CHANGELOG.md
index d13e308d8c..524175e326 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Changes
+
+* [#228](https://github.com/rubocop/rubocop-performance/pull/228): Mark `Performance/RedundantMerge` as unsafe. ([@dvandersluis][])
+
 ## 1.10.2 (2021-03-23)
 
 ### Bug fixes
diff --git a/config/default.yml b/config/default.yml
index 17f3e7c249..8f4feb984a 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -220,7 +220,9 @@ Performance/RedundantMerge:
   Description: 'Use Hash#[]=, rather than Hash#merge! with a single key-value pair.'
   Reference: 'https://github.com/JuanitoFatas/fast-ruby#hashmerge-vs-hash-code'
   Enabled: true
+  Safe: false
   VersionAdded: '0.36'
+  VersionChanged: '1.11'
   # Max number of key-value pairs to consider an offense
   MaxKeyValuePairs: 2
 
diff --git a/lib/rubocop/cop/performance/redundant_merge.rb b/lib/rubocop/cop/performance/redundant_merge.rb
index d986cc3a09..ee920ef50a 100644
--- a/lib/rubocop/cop/performance/redundant_merge.rb
+++ b/lib/rubocop/cop/performance/redundant_merge.rb
@@ -8,6 +8,9 @@ module Performance
       # You can set the maximum number of key-value pairs to consider
       # an offense with `MaxKeyValuePairs`.
       #
+      # This cop is marked as unsafe because RuboCop cannot determine if the
+      # receiver of `merge!` is actually a hash or not.
+      #
       # @example
       #   # bad
       #   hash.merge!(a: 1)