diff --git a/package.xml b/package.xml
index ecd920dfa5..e6ad2d7be0 100644
--- a/package.xml
+++ b/package.xml
@@ -381,6 +381,7 @@ http://pear.php.net/dtd/package-2.0.xsd">
         <file baseinstalldir="PHP/CodeSniffer" name="ForLoopShouldBeWhileLoopSniff.php" role="php" />
         <file baseinstalldir="PHP/CodeSniffer" name="ForLoopWithTestFunctionCallSniff.php" role="php" />
         <file baseinstalldir="PHP/CodeSniffer" name="JumbledIncrementerSniff.php" role="php" />
+        <file baseinstalldir="PHP/CodeSniffer" name="MixedBooleanOperatorSniff.php" role="php" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnconditionalIfStatementSniff.php" role="php" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnnecessaryFinalModifierSniff.php" role="php" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnusedFunctionParameterSniff.php" role="php" />
@@ -517,6 +518,8 @@ http://pear.php.net/dtd/package-2.0.xsd">
         <file baseinstalldir="PHP/CodeSniffer" name="ForLoopWithTestFunctionCallUnitTest.php" role="test" />
         <file baseinstalldir="PHP/CodeSniffer" name="JumbledIncrementerUnitTest.inc" role="test" />
         <file baseinstalldir="PHP/CodeSniffer" name="JumbledIncrementerUnitTest.php" role="test" />
+        <file baseinstalldir="PHP/CodeSniffer" name="MixedBooleanOperatorUnitTest.inc" role="test" />
+        <file baseinstalldir="PHP/CodeSniffer" name="MixedBooleanOperatorUnitTest.php" role="test" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnconditionalIfStatementUnitTest.inc" role="test" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnconditionalIfStatementUnitTest.php" role="test" />
         <file baseinstalldir="PHP/CodeSniffer" name="UnnecessaryFinalModifierUnitTest.inc" role="test" />
diff --git a/src/Standards/Generic/Sniffs/CodeAnalysis/MixedBooleanOperatorSniff.php b/src/Standards/Generic/Sniffs/CodeAnalysis/MixedBooleanOperatorSniff.php
new file mode 100644
index 0000000000..5fd9d113ba
--- /dev/null
+++ b/src/Standards/Generic/Sniffs/CodeAnalysis/MixedBooleanOperatorSniff.php
@@ -0,0 +1,104 @@
+<?php
+/**
+ * Detects mixed '&&' and '||' within a single expression, without making
+ * precedence explicit using parentheses.
+ *
+ * <code>
+ * $var = true && true || true;
+ * </code>
+ *
+ * @author    Tim Duesterhus <duesterhus@woltlab.com>
+ * @copyright 2021 WoltLab GmbH.
+ * @license   https://github.com/squizlabs/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
+ */
+
+namespace PHP_CodeSniffer\Standards\Generic\Sniffs\CodeAnalysis;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+class MixedBooleanOperatorSniff implements Sniff
+{
+
+
+    /**
+     * Registers the tokens that this sniff wants to listen for.
+     *
+     * @return int[]
+     */
+    public function register()
+    {
+        return [
+            T_BOOLEAN_OR,
+            T_BOOLEAN_AND,
+        ];
+
+    }//end register()
+
+
+    /**
+     * Processes this test, when one of its tokens is encountered.
+     *
+     * @param \PHP_CodeSniffer\Files\File $phpcsFile The file being scanned.
+     * @param int                         $stackPtr  The position of the current token
+     *                                               in the stack passed in $tokens.
+     *
+     * @return void
+     */
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        $tokens = $phpcsFile->getTokens();
+        $token  = $tokens[$stackPtr];
+
+        $start = $phpcsFile->findStartOfStatement($stackPtr);
+
+        if ($token['code'] === T_BOOLEAN_AND) {
+            $search = T_BOOLEAN_OR;
+        } else if ($token['code'] === T_BOOLEAN_OR) {
+            $search = T_BOOLEAN_AND;
+        } else {
+            throw new \LogicException('Unreachable');
+        }
+
+        while (true) {
+            $previous = $phpcsFile->findPrevious(
+                [
+                    $search,
+                    T_OPEN_PARENTHESIS,
+                    T_OPEN_SQUARE_BRACKET,
+                    T_CLOSE_PARENTHESIS,
+                    T_CLOSE_SQUARE_BRACKET,
+                ],
+                $stackPtr,
+                $start
+            );
+
+            if ($previous === false) {
+                break;
+            }
+
+            if ($tokens[$previous]['code'] === T_OPEN_PARENTHESIS
+                || $tokens[$previous]['code'] === T_OPEN_SQUARE_BRACKET
+            ) {
+                // We halt if we reach the opening parens / bracket of the boolean operator.
+                return;
+            } else if ($tokens[$previous]['code'] === T_CLOSE_PARENTHESIS) {
+                // We skip the content of nested parens.
+                $stackPtr = ($tokens[$previous]['parenthesis_opener'] - 1);
+            } else if ($tokens[$previous]['code'] === T_CLOSE_SQUARE_BRACKET) {
+                // We skip the content of nested brackets.
+                $stackPtr = ($tokens[$previous]['bracket_opener'] - 1);
+            } else if ($tokens[$previous]['code'] === $search) {
+                // We reached a mismatching operator, thus we must report the error.
+                $error = "Mixed '&&' and '||' within an expression without using parentheses.";
+                $phpcsFile->addError($error, $stackPtr, 'MissingParentheses', []);
+                return;
+            } else {
+                throw new \LogicException('Unreachable');
+            }
+        }//end while
+
+    }//end process()
+
+
+}//end class
diff --git a/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.inc b/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.inc
new file mode 100644
index 0000000000..cc88ecd189
--- /dev/null
+++ b/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.inc
@@ -0,0 +1,41 @@
+<?php
+
+if (true && true || true);
+if ((true && true) || true);
+if (true && (true || true));
+
+$var = true && true || true;
+$var = (true && true) || true;
+$var = true && (true || true);
+
+$complex = true && (true || true) && true;
+$complex = true && (true || true) || true;
+
+if (
+    true
+    && true
+    || true
+);
+
+if (
+    true
+    && (
+        true
+        || true
+    )
+);
+
+if (true && foo(true || true));
+if (true && foo(true && true || true));
+if (true && $foo[true || true]);
+if (true && $foo[true && true || true]);
+
+if (true && foo(true) || true);
+if (true && $foo[true] || true);
+if (true && foo($foo[true]) || true);
+
+$foo[] = true && true || false;
+
+foo([true && true || false]);
+
+if (true && true || true && true);
diff --git a/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.php b/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.php
new file mode 100644
index 0000000000..889d4c9ea0
--- /dev/null
+++ b/src/Standards/Generic/Tests/CodeAnalysis/MixedBooleanOperatorUnitTest.php
@@ -0,0 +1,61 @@
+<?php
+/**
+ * Unit test class for the MixedBooleanOperator sniff.
+ *
+ * @author    Tim Duesterhus <duesterhus@woltlab.com>
+ * @copyright 2021 WoltLab GmbH.
+ * @license   https://github.com/squizlabs/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
+ */
+
+namespace PHP_CodeSniffer\Standards\Generic\Tests\CodeAnalysis;
+
+use PHP_CodeSniffer\Tests\Standards\AbstractSniffUnitTest;
+
+class MixedBooleanOperatorUnitTest extends AbstractSniffUnitTest
+{
+
+
+    /**
+     * Returns the lines where errors should occur.
+     *
+     * The key of the array should represent the line number and the value
+     * should represent the number of errors that should occur on that line.
+     *
+     * @return array<int, int>
+     */
+    public function getErrorList()
+    {
+        return [
+            3  => 1,
+            7  => 1,
+            12 => 1,
+            17 => 1,
+            29 => 1,
+            31 => 1,
+            33 => 1,
+            34 => 1,
+            35 => 1,
+            37 => 1,
+            39 => 1,
+            41 => 2,
+        ];
+
+    }//end getErrorList()
+
+
+    /**
+     * Returns the lines where warnings should occur.
+     *
+     * The key of the array should represent the line number and the value
+     * should represent the number of warnings that should occur on that line.
+     *
+     * @return array<int, int>
+     */
+    public function getWarningList()
+    {
+        return [];
+
+    }//end getWarningList()
+
+
+}//end class