diff --git a/api/src/DTO/ResetPassword.php b/api/src/DTO/ResetPassword.php
index 3445e30fad..7bd261031a 100644
--- a/api/src/DTO/ResetPassword.php
+++ b/api/src/DTO/ResetPassword.php
@@ -51,6 +51,7 @@ class ResetPassword {
     public ?string $id = null;
 
     #[InputFilter\Trim]
+    #[InputFilter\Lowercase]
     #[ApiProperty(readable: true, writable: true)]
     #[Groups(['create', 'read'])]
     public ?string $email = null;
diff --git a/api/src/Entity/CampCollaboration.php b/api/src/Entity/CampCollaboration.php
index d26edbe1fb..72f6a11515 100644
--- a/api/src/Entity/CampCollaboration.php
+++ b/api/src/Entity/CampCollaboration.php
@@ -131,6 +131,7 @@ class CampCollaboration extends BaseEntity implements BelongsToCampInterface {
      * a user account. Either this field or the user field should be null.
      */
     #[InputFilter\Trim]
+    #[InputFilter\Lowercase]
     #[Assert\Email]
     #[Assert\Length(min: 1, max: 128)]
     #[AssertEitherIsNull(other: 'user')]
diff --git a/api/src/Entity/Profile.php b/api/src/Entity/Profile.php
index 536f67adba..52ccf5b924 100644
--- a/api/src/Entity/Profile.php
+++ b/api/src/Entity/Profile.php
@@ -53,6 +53,7 @@ class Profile extends BaseEntity {
      * Can only be changed by setting the newEmail field, which triggers an email verification flow.
      */
     #[InputFilter\Trim]
+    #[InputFilter\Lowercase]
     #[Assert\NotBlank]
     #[Assert\Email]
     #[ApiProperty(example: self::EXAMPLE_EMAIL)]
@@ -65,6 +66,7 @@ class Profile extends BaseEntity {
      * If set, a verification email is sent to this email address.
      */
     #[InputFilter\Trim]
+    #[InputFilter\Lowercase]
     #[Assert\Email]
     #[ApiProperty(example: self::EXAMPLE_EMAIL)]
     #[Groups(['write'])]
diff --git a/api/src/InputFilter/Lowercase.php b/api/src/InputFilter/Lowercase.php
new file mode 100644
index 0000000000..ebb402ac91
--- /dev/null
+++ b/api/src/InputFilter/Lowercase.php
@@ -0,0 +1,13 @@
+<?php
+
+namespace App\InputFilter;
+
+/**
+ * Lowercases a string using the PHP strtolower() function.
+ */
+#[\Attribute(\Attribute::TARGET_PROPERTY | \Attribute::IS_REPEATABLE)]
+class Lowercase extends FilterAttribute {
+    public function __construct(array $options = [], int $priority = 10) {
+        parent::__construct($options, $priority);
+    }
+}
diff --git a/api/src/InputFilter/LowercaseFilter.php b/api/src/InputFilter/LowercaseFilter.php
new file mode 100644
index 0000000000..e5255838e0
--- /dev/null
+++ b/api/src/InputFilter/LowercaseFilter.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace App\InputFilter;
+
+class LowercaseFilter extends InputFilter {
+    /**
+     * {@inheritdoc}
+     */
+    public function applyTo(array $data, string $propertyName): array {
+        if (!array_key_exists($propertyName, $data)) {
+            return $data;
+        }
+
+        $value = $data[$propertyName];
+
+        if (null === $value) {
+            return $data;
+        }
+
+        if (!is_scalar($value) && !(\is_object($value) && method_exists($value, '__toString'))) {
+            throw new UnexpectedValueException('Cannot convert value to string for lowercasing.');
+        }
+
+        $value = (string) $value;
+
+        $data[$propertyName] = strtolower($value);
+
+        return $data;
+    }
+}
diff --git a/api/src/Security/OAuth/GoogleAuthenticator.php b/api/src/Security/OAuth/GoogleAuthenticator.php
index b03e8a72e2..9d2b1a88af 100644
--- a/api/src/Security/OAuth/GoogleAuthenticator.php
+++ b/api/src/Security/OAuth/GoogleAuthenticator.php
@@ -57,7 +57,8 @@ public function authenticate(Request $request): Passport {
                 }
 
                 // do we have a matching user by email?
-                $profile = $profileRepository->findOneBy(['email' => $email]);
+                $profiles = $profileRepository->findBy(['email' => strtolower($email)]);
+                $profile = count($profiles) > 0 ? $profiles[0] : null;
                 $user = $profile?->user;
 
                 if (is_null($profile)) {
diff --git a/api/src/Security/OAuth/HitobitoAuthenticator.php b/api/src/Security/OAuth/HitobitoAuthenticator.php
index 7b814e570e..85a350404b 100644
--- a/api/src/Security/OAuth/HitobitoAuthenticator.php
+++ b/api/src/Security/OAuth/HitobitoAuthenticator.php
@@ -63,7 +63,8 @@ public function authenticate(Request $request): Passport {
                 }
 
                 // do we have a matching user by email?
-                $profile = $profileRepository->findOneBy(['email' => $email]);
+                $profiles = $profileRepository->findBy(['email' => strtolower($email)]);
+                $profile = count($profiles) > 0 ? $profiles[0] : null;
                 $user = $profile?->user;
 
                 if (is_null($profile)) {
diff --git a/api/tests/Api/CampCollaborations/CreateCampCollaborationTest.php b/api/tests/Api/CampCollaborations/CreateCampCollaborationTest.php
index 527af5f829..e37432115e 100644
--- a/api/tests/Api/CampCollaborations/CreateCampCollaborationTest.php
+++ b/api/tests/Api/CampCollaborations/CreateCampCollaborationTest.php
@@ -257,6 +257,29 @@ public function testCreateCampCollaborationTrimsInviteEmail() {
         ]));
     }
 
+    public function testCreateCampCollaborationLowercasesInviteEmail(): void {
+        static::createClientWithCredentials()->request(
+            'POST',
+            '/camp_collaborations',
+            [
+                'json' => $this->getExampleWritePayload(
+                    [
+                        'inviteEmail' => 'sOmeonE@example.COM',
+                    ],
+                    ['user']
+                ),
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertJsonContains($this->getExampleReadPayload([
+            'inviteEmail' => 'someone@example.com',
+            '_links' => [
+                'user' => null,
+            ],
+        ]));
+    }
+
     public function testCreateCampCollaborationWithInviteEmailInsteadOfUserIsPossible() {
         static::createClientWithCredentials()->request('POST', '/camp_collaborations', ['json' => $this->getExampleWritePayload([
             'inviteEmail' => 'someone@example.com',
diff --git a/api/tests/Api/ResetPassword/ResetPasswordTest.php b/api/tests/Api/ResetPassword/ResetPasswordTest.php
index 5e5d087a25..882fc7ea80 100644
--- a/api/tests/Api/ResetPassword/ResetPasswordTest.php
+++ b/api/tests/Api/ResetPassword/ResetPasswordTest.php
@@ -107,6 +107,24 @@ public function testPostResetPasswordTrimsEmail() {
         self::assertEmailCount(1);
     }
 
+    public function testPostResetPasswordLowercasesEmail() {
+        /** @var User $user */
+        $user = static::getFixture('user1manager');
+
+        $this->createBasicClient()->request(
+            'POST',
+            '/auth/reset_password',
+            [
+                'json' => [
+                    'email' => strtoupper("{$user->getEmail()}"),
+                ],
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(204);
+        self::assertEmailCount(1);
+    }
+
     public function testPostResetPasswordReturns204ForUnknownEmailButSendsNoEmails() {
         $this->createBasicClient()->request(
             'POST',
diff --git a/api/tests/Api/Users/CreateUserTest.php b/api/tests/Api/Users/CreateUserTest.php
index bc22473620..7b422b4ccd 100644
--- a/api/tests/Api/Users/CreateUserTest.php
+++ b/api/tests/Api/Users/CreateUserTest.php
@@ -208,6 +208,34 @@ public function testCreateUserTrimsEmail() {
         ));
     }
 
+    public function testCreateUserLowercaseEmail(): void {
+        static::createBasicClient()->request(
+            'POST',
+            '/users',
+            [
+                'json' => $this->getExampleWritePayload(
+                    mergeEmbeddedAttributes: [
+                        'profile' => [
+                            'email' => 'Bi-pi@example.COM',
+                        ],
+                    ]
+                ),
+            ]
+        );
+
+        $this->assertResponseStatusCodeSame(201);
+        $this->assertJsonContains($this->getExampleReadPayload(
+            [
+                '_embedded' => [
+                    'profile' => [
+                        'email' => 'bi-pi@example.com',
+                    ],
+                ],
+            ],
+            ['password']
+        ));
+    }
+
     public function testCreateUserValidatesMissingEmail() {
         // use this easy way here, because unsetting a nested attribute would be complicated
         $exampleWritePayload = $this->getExampleWritePayload();
diff --git a/api/tests/InputFilter/LowercaseFilterTest.php b/api/tests/InputFilter/LowercaseFilterTest.php
new file mode 100644
index 0000000000..14554f4eda
--- /dev/null
+++ b/api/tests/InputFilter/LowercaseFilterTest.php
@@ -0,0 +1,77 @@
+<?php
+
+namespace App\Tests\InputFilter;
+
+use App\InputFilter\LowercaseFilter;
+use App\InputFilter\UnexpectedValueException;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * @internal
+ */
+class LowercaseFilterTest extends TestCase {
+    #[DataProvider('getExamples')]
+    public function testInputFiltering(string $input, string $output): void {
+        // given
+        $data = ['key' => $input];
+        $outputData = ['key' => $output];
+        $trim = new LowercaseFilter();
+
+        // when
+        $result = $trim->applyTo($data, 'key');
+
+        // then
+        $this->assertEquals($outputData, $result);
+    }
+
+    public static function getExamples(): array {
+        return [
+            ['', ''],
+            ['abc', 'abc'],
+            ['AbC', 'abc'],
+            ['This Is A Test', 'this is a test'],
+            ['TeSt123', 'test123'],
+            ['Test@Example.com', 'test@example.com'],
+            ['JohnDoe@GMail.COM', 'johndoe@gmail.com'],
+            ['USER-NAME+TAG@EXAMPLE.NET', 'user-name+tag@example.net'],
+            ['info@sub.domain.COM', 'info@sub.domain.com'],
+        ];
+    }
+
+    public function testDoesNothingWhenKeyIsMissing(): void {
+        // given
+        $data = ['otherkey' => 'something'];
+        $lowercase = new LowercaseFilter();
+
+        // when
+        $result = $lowercase->applyTo($data, 'key');
+
+        // then
+        $this->assertEquals($data, $result);
+    }
+
+    public function testDoesNothingWhenValueIsNull(): void {
+        // given
+        $data = ['key' => null];
+        $trim = new LowercaseFilter();
+
+        // when
+        $result = $trim->applyTo($data, 'key');
+
+        // then
+        $this->assertEquals($data, $result);
+    }
+
+    public function testThrowsWhenValueIsNotStringable(): void {
+        // given
+        $data = ['key' => new \stdClass()];
+        $trim = new LowercaseFilter();
+
+        // then
+        $this->expectException(UnexpectedValueException::class);
+
+        // when
+        $trim->applyTo($data, 'key');
+    }
+}