diff --git a/CHANGELOG.md b/CHANGELOG.md
index 41db0807bd..df8e78e0a7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Changes
+
+* [#149](/~https://github.com/rubocop-hq/rubocop-performance/pull/149): Mark `Performance/AncestorsInclude` as unsafe. ([@eugeneius][])
+
 ## 1.7.0 (2020-07-07)
 
 ### New features
diff --git a/config/default.yml b/config/default.yml
index cd2acdbbc3..00618564e0 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -4,6 +4,7 @@ Performance/AncestorsInclude:
   Description: 'Use `A <= B` instead of `A.ancestors.include?(B)`.'
   Reference: '/~https://github.com/JuanitoFatas/fast-ruby#ancestorsinclude-vs--code'
   Enabled: 'pending'
+  Safe: false
   VersionAdded: '1.7'
 
 Performance/BigDecimalWithNumericArgument:
diff --git a/docs/modules/ROOT/pages/cops_performance.adoc b/docs/modules/ROOT/pages/cops_performance.adoc
index c95695c125..61c786f132 100644
--- a/docs/modules/ROOT/pages/cops_performance.adoc
+++ b/docs/modules/ROOT/pages/cops_performance.adoc
@@ -6,8 +6,8 @@
 | Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
 
 | Pending
-| Yes
-| Yes
+| No
+| Yes (Unsafe)
 | 1.7
 | -
 |===