diff --git a/CHANGELOG.md b/CHANGELOG.md
index d13e308..524175e 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 17f3e7c..8f4feb9 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/docs/modules/ROOT/pages/cops_performance.adoc b/docs/modules/ROOT/pages/cops_performance.adoc
index 3aebd6a..0f0aa67 100644
--- a/docs/modules/ROOT/pages/cops_performance.adoc
+++ b/docs/modules/ROOT/pages/cops_performance.adoc
@@ -1272,10 +1272,10 @@ return value unless regex =~ 'str'
 | Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
 
 | Enabled
-| Yes
-| Yes
+| No
+| Yes (Unsafe)
 | 0.36
-| -
+| 1.11
 |===
 
 This cop identifies places where `Hash#merge!` can be replaced by
@@ -1283,6 +1283,9 @@ This cop identifies places where `Hash#merge!` can be replaced by
 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.
+
 === Examples
 
 [source,ruby]
diff --git a/lib/rubocop/cop/performance/redundant_merge.rb b/lib/rubocop/cop/performance/redundant_merge.rb
index d986cc3..ee920ef 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)