diff --git a/src/Ruleset.php b/src/Ruleset.php
index b6279bf9f3..ded66b4c9b 100644
--- a/src/Ruleset.php
+++ b/src/Ruleset.php
@@ -1455,6 +1455,19 @@ public function populateTokenListeners()
             $this->sniffs[$sniffClass] = new $sniffClass();
 
             $sniffCode = Common::getSniffCode($sniffClass);
+
+            if (substr($sniffCode, 0, 1) === '.'
+                || substr($sniffCode, -1) === '.'
+                || strpos($sniffCode, '..') !== false
+                || preg_match('`(^|\.)Sniffs\.`', $sniffCode) === 1
+                || preg_match('`[^\s\.-]+\\\\Sniffs\\\\[^\s\.-]+\\\\[^\s\.-]+Sniff`', $sniffClass) !== 1
+            ) {
+                $message  = "The sniff $sniffClass does not comply with the PHP_CodeSniffer naming conventions.";
+                $message .= ' This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+                $message .= 'Contact the sniff author to fix the sniff.';
+                $this->msgCache->add($message, MessageCollector::DEPRECATED);
+            }
+
             $this->sniffCodes[$sniffCode] = $sniffClass;
 
             if ($this->sniffs[$sniffClass] instanceof DeprecatedSniff) {
diff --git a/tests/Core/Ruleset/ExpandSniffDirectoryTest.php b/tests/Core/Ruleset/ExpandSniffDirectoryTest.php
index 8f7f4f93f6..ddd3a51489 100644
--- a/tests/Core/Ruleset/ExpandSniffDirectoryTest.php
+++ b/tests/Core/Ruleset/ExpandSniffDirectoryTest.php
@@ -49,20 +49,9 @@ public function testExpandSniffDirectory()
         $this->assertNotFalse($expectedPathToRuleset, 'Ruleset file could not be found');
         $this->assertContains($expectedPathToRuleset, $ruleset->paths, 'Ruleset file not included in the "seen ruleset paths"');
 
-        /*
-         * Take note: the expectation includes some "undesirables" related to the convoluted directory structure
-         * in the "standard" used as a test fixture.
-         *
-         * That is okay as (for now) non-standard directory layouts are supported.
-         *
-         * This test is not about the standard directory layout.
-         */
-
         $expectedSniffCodes = [
-            '.Sniffs.IncorrectLevelShouldStillBeFound'       => 'MyStandard\\Sniffs\\IncorrectLevelShouldStillBeFoundSniff',
-            'MyStandard.CategoryA.FindMe'                    => 'MyStandard\\Sniffs\\CategoryA\\FindMeSniff',
-            'MyStandard.CategoryB.FindMe'                    => 'MyStandard\\Sniffs\\CategoryB\\FindMeSniff',
-            'Sniffs.SubDir.IncorrectLevelShouldStillBeFound' => 'MyStandard\\Sniffs\\CategoryA\\SubDir\\IncorrectLevelShouldStillBeFoundSniff',
+            'MyStandard.CategoryA.FindMe' => 'MyStandard\\Sniffs\\CategoryA\\FindMeSniff',
+            'MyStandard.CategoryB.FindMe' => 'MyStandard\\Sniffs\\CategoryB\\FindMeSniff',
         ];
 
         // Sort the value to make the tests stable as different OSes will read directories
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/Sniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/Sniff.php
new file mode 100644
index 0000000000..9da190d175
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/Sniff.php
@@ -0,0 +1,24 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\RulesetPopulateTokenListenersNamingConventionsTest
+ */
+
+namespace BrokenNamingConventions\Sniffs\Category;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff as PHPCS_Sniff;
+
+final class Sniff implements PHPCS_Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/SubDir/TooDeeplyNestedSniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/SubDir/TooDeeplyNestedSniff.php
new file mode 100644
index 0000000000..abba3557ca
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Category/SubDir/TooDeeplyNestedSniff.php
@@ -0,0 +1,24 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\RulesetPopulateTokenListenersNamingConventionsTest
+ */
+
+namespace BrokenNamingConventions\Sniffs\Category\SubDir;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+final class TooDeeplyNestedSniff implements Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/MissingCategoryDirSniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/MissingCategoryDirSniff.php
new file mode 100644
index 0000000000..57fee19ac9
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/MissingCategoryDirSniff.php
@@ -0,0 +1,24 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\RulesetPopulateTokenListenersNamingConventionsTest
+ */
+
+namespace BrokenNamingConventions\Sniffs;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+final class MissingCategoryDirSniff implements Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/NoNamespaceSniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/NoNamespaceSniff.php
new file mode 100644
index 0000000000..85dc940fe2
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/NoNamespaceSniff.php
@@ -0,0 +1,22 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\Ruleset\PopulateTokenListenersNamingConventionsTest
+ */
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+final class NoNamespaceSniff implements Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/PartialNamespaceSniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/PartialNamespaceSniff.php
new file mode 100644
index 0000000000..77bb14693f
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/PartialNamespaceSniff.php
@@ -0,0 +1,24 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\Ruleset\PopulateTokenListenersNamingConventionsTest
+ */
+
+namespace Sniffs;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+final class PartialNamespaceSniff implements Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Sniffs/CategoryCalledSniffsSniff.php b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Sniffs/CategoryCalledSniffsSniff.php
new file mode 100644
index 0000000000..7312e80e8e
--- /dev/null
+++ b/tests/Core/Ruleset/Fixtures/BrokenNamingConventions/Sniffs/Sniffs/CategoryCalledSniffsSniff.php
@@ -0,0 +1,24 @@
+<?php
+/**
+ * Test fixture.
+ *
+ * @see \PHP_CodeSniffer\Tests\Core\RulesetPopulateTokenListenersNamingConventionsTest
+ */
+
+namespace BrokenNamingConventions\Sniffs\Sniffs;
+
+use PHP_CodeSniffer\Files\File;
+use PHP_CodeSniffer\Sniffs\Sniff;
+
+final class CategoryCalledSniffsSniff implements Sniff
+{
+    public function register()
+    {
+        return [T_OPEN_TAG];
+    }
+
+    public function process(File $phpcsFile, $stackPtr)
+    {
+        // Do something.
+    }
+}
diff --git a/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/CategoryA/Subdir/IncorrectLevelShouldStillBeFoundSniff.php b/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/CategoryA/Subdir/IncorrectLevelShouldStillBeFoundSniff.php
deleted file mode 100644
index 734fb932c9..0000000000
--- a/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/CategoryA/Subdir/IncorrectLevelShouldStillBeFoundSniff.php
+++ /dev/null
@@ -1,12 +0,0 @@
-<?php
-/**
- * Test fixture.
- *
- * @see \PHP_CodeSniffer\Tests\Core\Ruleset\ExpandSniffDirectoryTest
- */
-
-namespace MyStandard\Sniffs\CategoryA\SubDir;
-
-use MyStandard\DummySniff;
-
-final class IncorrectLevelShouldStillBeFoundSniff extends DummySniff {}
diff --git a/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/IncorrectLevelShouldStillBeFoundSniff.php b/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/IncorrectLevelShouldStillBeFoundSniff.php
deleted file mode 100644
index c5d05653cd..0000000000
--- a/tests/Core/Ruleset/Fixtures/DirectoryExpansion/.hiddenAbove/src/MyStandard/Sniffs/IncorrectLevelShouldStillBeFoundSniff.php
+++ /dev/null
@@ -1,12 +0,0 @@
-<?php
-/**
- * Test fixture.
- *
- * @see \PHP_CodeSniffer\Tests\Core\Ruleset\ExpandSniffDirectoryTest
- */
-
-namespace MyStandard\Sniffs;
-
-use MyStandard\DummySniff;
-
-final class IncorrectLevelShouldStillBeFoundSniff extends DummySniff {}
diff --git a/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.php b/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.php
new file mode 100644
index 0000000000..d93840521b
--- /dev/null
+++ b/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.php
@@ -0,0 +1,82 @@
+<?php
+/**
+ * Test the Ruleset::expandSniffDirectory() method.
+ *
+ * @author    Juliette Reinders Folmer <phpcs_nospam@adviesenzo.nl>
+ * @copyright 2024 PHPCSStandards and contributors
+ * @license   https://github.com/PHPCSStandards/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
+ */
+
+namespace PHP_CodeSniffer\Tests\Core\Ruleset;
+
+use PHP_CodeSniffer\Ruleset;
+use PHP_CodeSniffer\Tests\ConfigDouble;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Test handling of sniffs not following the PHPCS naming conventions in the Ruleset::populateTokenListeners() method.
+ *
+ * @covers \PHP_CodeSniffer\Ruleset::populateTokenListeners
+ */
+final class PopulateTokenListenersNamingConventionsTest extends TestCase
+{
+
+
+    /**
+     * Verify a warning is shown for sniffs not complying with the PHPCS naming conventions.
+     *
+     * Including sniffs which do not comply with the PHPCS naming conventions is soft deprecated since
+     * PHPCS 3.12.0, hard deprecated since PHPCS 3.13.0 and support will be removed in PHPCS 4.0.0.
+     *
+     * @return void
+     */
+    public function testBrokenNamingConventions()
+    {
+        // Set up the ruleset.
+        $standard = __DIR__.'/PopulateTokenListenersNamingConventionsTest.xml';
+        $config   = new ConfigDouble(["--standard=$standard"]);
+        $ruleset  = new Ruleset($config);
+
+        // The "Generic.PHP.BacktickOperator" sniff is the only valid sniff.
+        $expectedSniffCodes = [
+            '..NoNamespace'                                       => 'NoNamespaceSniff',
+            '.Sniffs.MissingCategoryDir'                          => 'BrokenNamingConventions\\Sniffs\\MissingCategoryDirSniff',
+            '.Sniffs.PartialNamespace'                            => 'Sniffs\\PartialNamespaceSniff',
+            'BrokenNamingConventions.Category.'                   => 'BrokenNamingConventions\\Sniffs\\Category\\Sniff',
+            'BrokenNamingConventions.Sniffs.CategoryCalledSniffs' => 'BrokenNamingConventions\\Sniffs\\Sniffs\\CategoryCalledSniffsSniff',
+            'Generic.PHP.BacktickOperator'                        => 'PHP_CodeSniffer\\Standards\\Generic\\Sniffs\\PHP\\BacktickOperatorSniff',
+            'Sniffs.SubDir.TooDeeplyNested'                       => 'BrokenNamingConventions\\Sniffs\\Category\\SubDir\\TooDeeplyNestedSniff',
+        ];
+
+        // Sort the value to make the tests stable as different OSes will read directories
+        // in a different order and the order is not relevant for these tests. Just the values.
+        $actual = $ruleset->sniffCodes;
+        ksort($actual);
+
+        $this->assertSame($expectedSniffCodes, $actual, 'Registered sniffs do not match expectation');
+
+        $expectedMessage  = 'DEPRECATED: The sniff BrokenNamingConventions\\Sniffs\\MissingCategoryDirSniff does not comply';
+        $expectedMessage .= ' with the PHP_CodeSniffer naming conventions. This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL;
+        $expectedMessage .= 'DEPRECATED: The sniff NoNamespaceSniff does not comply with the PHP_CodeSniffer naming conventions.';
+        $expectedMessage .= ' This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL;
+        $expectedMessage .= 'DEPRECATED: The sniff Sniffs\\PartialNamespaceSniff does not comply with the PHP_CodeSniffer naming conventions.';
+        $expectedMessage .= ' This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL;
+        $expectedMessage .= 'DEPRECATED: The sniff BrokenNamingConventions\\Sniffs\\Category\\Sniff does not comply';
+        $expectedMessage .= ' with the PHP_CodeSniffer naming conventions. This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL;
+        $expectedMessage .= 'DEPRECATED: The sniff BrokenNamingConventions\\Sniffs\\Sniffs\\CategoryCalledSniffsSniff does not';
+        $expectedMessage .= ' comply with the PHP_CodeSniffer naming conventions. This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL;
+        $expectedMessage .= 'DEPRECATED: The sniff BrokenNamingConventions\\Sniffs\\Category\\SubDir\\TooDeeplyNestedSniff';
+        $expectedMessage .= ' does not comply with the PHP_CodeSniffer naming conventions. This will no longer be supported in PHPCS 4.0.'.PHP_EOL;
+        $expectedMessage .= 'Contact the sniff author to fix the sniff.'.PHP_EOL.PHP_EOL;
+
+        $this->expectOutputString($expectedMessage);
+
+    }//end testBrokenNamingConventions()
+
+
+}//end class
diff --git a/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.xml b/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.xml
new file mode 100644
index 0000000000..4446c0dce8
--- /dev/null
+++ b/tests/Core/Ruleset/PopulateTokenListenersNamingConventionsTest.xml
@@ -0,0 +1,24 @@
+<?xml version="1.0"?>
+<ruleset xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" name="PopulateTokenListenersNamingConventionsTest" xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/PHPCSStandards/PHP_CodeSniffer/master/phpcs.xsd">
+
+    <!--
+    These sniff files are not in an installed standard, there is not even a ruleset.xml file for the "standard".
+    Each of these sniff files breaks one of the naming convention rules.
+    -->
+
+    <!-- Breaks: All sniffs MUST be located within a [CategoryName] directory. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/MissingCategoryDirSniff.php"/>
+    <!-- Breaks: The namespace and class name MUST follow PSR-4. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/NoNamespaceSniff.php"/>
+    <!-- Breaks: The namespace and class name MUST follow PSR-4. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/PartialNamespaceSniff.php"/>
+    <!-- Breaks: All sniffs MUST have a name, so a sniff class called just and only Sniff is not allowed. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/Category/Sniff.php"/>
+    <!-- Breaks: The name "Sniffs" MUST NOT be used as a category name. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/Sniffs/CategoryCalledSniffsSniff.php"/>
+    <!-- Breaks: No directories should exist under the [CategoryName] directory. -->
+    <rule ref="./Fixtures/BrokenNamingConventions/Sniffs/Category/SubDir/TooDeeplyNestedSniff.php"/>
+
+    <!-- Prevent a "no sniff were registered" error (once the above become errors). -->
+    <rule ref="Generic.PHP.BacktickOperator"/>
+</ruleset>