diff --git a/STYLEGUIDE.md b/STYLEGUIDE.md
index 6cb942d1..31f33664 100644
--- a/STYLEGUIDE.md
+++ b/STYLEGUIDE.md
@@ -3,6 +3,7 @@
 This is GitHub's Ruby Style Guide, inspired by [RuboCop's guide][rubocop-guide].
 
 ## Table of Contents
+
 1. [Layout](#layout)
    1. [Indentation](#indentation)
    2. [Inline](#inline)
@@ -763,6 +764,22 @@ if x > 10
 end
 ```
 
+* Don't use `unless` with a negated condition.
+  <a name="no-unless-negation"></a><sup>[[link](#no-unless-negation)]</sup>
+  * <a href="https://docs.rubocop.org/rubocop/cops_style.html#stylenegatedunless">RuboCop rule: Style/NegatedUnless</a>
+
+```ruby
+# bad
+unless !condition?
+  do_something
+end
+
+# good
+if condition?
+  do_something
+end
+```
+
 ### Ternary operator
 
 * Avoid the ternary operator (`?:`) except in cases where all expressions are extremely
diff --git a/config/default.yml b/config/default.yml
index cf5d193e..115d24d6 100644
--- a/config/default.yml
+++ b/config/default.yml
@@ -1370,7 +1370,7 @@ Style/NegatedIfElseCondition:
   Enabled: false
 
 Style/NegatedUnless:
-  Enabled: false
+  Enabled: true
 
 Style/NegatedWhile:
   Enabled: false