diff --git a/packages/less/lib/less/parser/parser.js b/packages/less/lib/less/parser/parser.js
index 72014aab2..6a439fb89 100644
--- a/packages/less/lib/less/parser/parser.js
+++ b/packages/less/lib/less/parser/parser.js
@@ -2438,8 +2438,18 @@ const Parser = function Parser(context, imports, fileInfo, currentIndex) {
                     const result = this.parenthesisCondition(needsParens);
                     if (result) {
                         result.negate = !result.negate;
+                        return result;
+                    }
+
+                    // Allow simple bare values (keyword/variable) without parens,
+                    // e.g., `not false` or `not @var`.
+                    // Complex conditions (comparisons, function calls) require parentheses.
+                    const entities = this.entities;
+                    const index = parserInput.i;
+                    const a = entities.keyword() || entities.variable() || entities.quoted() || entities.mixinLookup();
+                    if (a) {
+                        return new(tree.Condition)('=', a, new(tree.Keyword)('true'), index + currentIndex, true);
                     }
-                    return result;
                 }
             },
             parenthesisCondition: function (needsParens) {
diff --git a/packages/test-data/tests-unit/functions/functions.css b/packages/test-data/tests-unit/functions/functions.css
index 4876f5874..17a990259 100644
--- a/packages/test-data/tests-unit/functions/functions.css
+++ b/packages/test-data/tests-unit/functions/functions.css
@@ -224,6 +224,8 @@ html {
   a: true;
   b: false;
   c: false;
+  d: true;
+  e: false;
 }
 #if {
   a: 1;
@@ -236,6 +238,8 @@ html {
   i: 6;
   j: 8;
   k: 1;
+  m: 1;
+  n: 2;
   l: black;
   /* results in void */
   color: green;
diff --git a/packages/test-data/tests-unit/functions/functions.less b/packages/test-data/tests-unit/functions/functions.less
index bc476b6e2..f11b756d2 100644
--- a/packages/test-data/tests-unit/functions/functions.less
+++ b/packages/test-data/tests-unit/functions/functions.less
@@ -256,6 +256,9 @@ html {
   a: boolean(not(2 < 1));
   b: boolean(not(2 > 1) and (true));
   c: boolean(not(boolean(true)));
+  // not without parentheses (should behave the same as with parentheses)
+  d: boolean(not false);
+  e: boolean(not true);
 }
 
 #if {
@@ -271,6 +274,9 @@ html {
   i: if(true and isnumber(6), 6, 8);
   j: if(not(true) and true, 6, 8);
   k: if(true or true, 1);
+  // not without parentheses
+  m: if(not false, 1, 2);
+  n: if(not true, 1, 2);
 
   // see: https://github.com/less/less.js/issues/3371
   @some: foo;
diff --git a/packages/test-data/tests-unit/mixins-guards/mixins-guards.css b/packages/test-data/tests-unit/mixins-guards/mixins-guards.css
index c54eca77e..fa164dd18 100644
--- a/packages/test-data/tests-unit/mixins-guards/mixins-guards.css
+++ b/packages/test-data/tests-unit/mixins-guards/mixins-guards.css
@@ -209,3 +209,9 @@
   no-parenthesis: evaluated true 4;
   with-parenthesis: evaluated true;
 }
+.test-not-noparens1 {
+  content: "not without parens true.";
+}
+.test-not-noparens2 {
+  content: "not without parens false.";
+}
diff --git a/packages/test-data/tests-unit/mixins-guards/mixins-guards.less b/packages/test-data/tests-unit/mixins-guards/mixins-guards.less
index 834c57d2c..c52749acb 100644
--- a/packages/test-data/tests-unit/mixins-guards/mixins-guards.less
+++ b/packages/test-data/tests-unit/mixins-guards/mixins-guards.less
@@ -356,3 +356,14 @@
   .orderOfEvaluation(true, true, false);
 }
 
+// not without parentheses should work the same as not with parentheses
+.test-not-noparens (@a) when not @a {
+    content: "not without parens false.";
+}
+.test-not-noparens (@a) when (@a) {
+    content: "not without parens true.";
+}
+
+.test-not-noparens1 { .test-not-noparens(true) }
+.test-not-noparens2 { .test-not-noparens(false) }
+