diff --git a/rules-tests/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector/Fixture/skip_constant_definition.php.inc b/rules-tests/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector/Fixture/skip_constant_definition.php.inc
new file mode 100644
index 00000000000..eee0f3d7dda
--- /dev/null
+++ b/rules-tests/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector/Fixture/skip_constant_definition.php.inc
@@ -0,0 +1,6 @@
+<?php
+
+namespace Rector\Tests\CodeQuality\Rector\Expression\InlineIfToExplicitIfRector\Fixture;
+
+defined('SHOW_DEBUG_BACKTRACE') || define('SHOW_DEBUG_BACKTRACE', true);
+! defined('SHOW_DEBUG_BACKTRACE') && define('SHOW_DEBUG_BACKTRACE', true);
diff --git a/rules/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector.php b/rules/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector.php
index cfb7be83a8d..ba8d70bbf07 100644
--- a/rules/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector.php
+++ b/rules/CodeQuality/Rector/Expression/InlineIfToExplicitIfRector.php
@@ -8,6 +8,8 @@
 use PhpParser\Node\Expr;
 use PhpParser\Node\Expr\BinaryOp\BooleanAnd;
 use PhpParser\Node\Expr\BinaryOp\BooleanOr;
+use PhpParser\Node\Expr\BooleanNot;
+use PhpParser\Node\Expr\FuncCall;
 use PhpParser\Node\Stmt\Expression;
 use PhpParser\Node\Stmt\If_;
 use PHPStan\Type\BooleanType;
@@ -93,6 +95,14 @@ private function processExplicitIf(Expression $expression): ?Node
             return null;
         }
 
+        $exprLeft = $booleanExpr->left instanceof BooleanNot
+            ? $booleanExpr->left->expr
+            : $booleanExpr->left;
+
+        if ($exprLeft instanceof FuncCall && $this->isName($exprLeft, 'defined')) {
+            return null;
+        }
+
         /** @var Expr $expr */
         $expr = $booleanExpr instanceof BooleanAnd
             ? $booleanExpr->left