diff --git a/CHANGELOG.md b/CHANGELOG.md
index be369485fe..3545855977 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@
 * [#312](https://github.com/rubocop-hq/rubocop-rails/pull/312): Mark `Rails/MailerName` as unsafe for auto-correct. ([@eugeneius][])
 * [#294](https://github.com/rubocop-hq/rubocop-rails/pull/294): Update `Rails/ReversibleMigration` to register offenses for `remove_columns` and `remove_index`. ([@philcoggins][])
 * [#310](https://github.com/rubocop-hq/rubocop-rails/issues/310): Add `EnforcedStyle` to `Rails/PluckInWhere`. By default, it does not register an offense if `pluck` method's receiver is a variable. ([@koic][])
+* [#320](https://github.com/rubocop-hq/rubocop-rails/pull/320): Mark `Rails/UniqBeforePluck` as unsafe. ([@kunitoo][])
 
 ## 2.7.1 (2020-07-26)
 
@@ -262,3 +263,4 @@
 [@taylorthurlow]: https://github.com/taylorthurlow
 [@krim]: https://github.com/krim
 [@philcoggins]: https://github.com/philcoggins
+[@kunitoo]: https://github.com/kunitoo
diff --git a/config/default.yml b/config/default.yml
index 2da68ebc49..3795e63a0f 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -657,11 +657,12 @@ Rails/UniqBeforePluck:
   Description: 'Prefer the use of uniq or distinct before pluck.'
   Enabled: true
   VersionAdded: '0.40'
-  VersionChanged: '2.6'
+  VersionChanged: '2.8'
   EnforcedStyle: conservative
   SupportedStyles:
     - conservative
     - aggressive
+  Safe: false
   AutoCorrect: false
 
 Rails/UniqueValidationWithoutIndex:
diff --git a/docs/modules/ROOT/pages/cops_rails.adoc b/docs/modules/ROOT/pages/cops_rails.adoc
index 5ef235d780..dc2c540cf8 100644
--- a/docs/modules/ROOT/pages/cops_rails.adoc
+++ b/docs/modules/ROOT/pages/cops_rails.adoc
@@ -3977,10 +3977,10 @@ Time.at(timestamp).in_time_zone
 | Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
 
 | Enabled
-| Yes
-| Yes
+| No
+| Yes (Unsafe)
 | 0.40
-| 2.6
+| 2.8
 |===
 
 Prefer the use of distinct, before pluck instead of after.
@@ -3998,6 +3998,8 @@ as the cop cannot distinguish between calls to pluck on an
 ActiveRecord::Relation vs a call to pluck on an
 ActiveRecord::Associations::CollectionProxy.
 
+This cop is unsafe because the behavior may change depending on the
+database collation.
 Autocorrect is disabled by default for this cop since it may generate
 false positives.
 
diff --git a/lib/rubocop/cop/rails/uniq_before_pluck.rb b/lib/rubocop/cop/rails/uniq_before_pluck.rb
index a650605842..c74d3f6d2d 100644
--- a/lib/rubocop/cop/rails/uniq_before_pluck.rb
+++ b/lib/rubocop/cop/rails/uniq_before_pluck.rb
@@ -18,6 +18,8 @@ module Rails
       # ActiveRecord::Relation vs a call to pluck on an
       # ActiveRecord::Associations::CollectionProxy.
       #
+      # This cop is unsafe because the behavior may change depending on the
+      # database collation.
       # Autocorrect is disabled by default for this cop since it may generate
       # false positives.
       #