diff --git a/CHANGELOG.md b/CHANGELOG.md
index b79f7913477..cbd932b0a68 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -43,6 +43,7 @@
 * [BUGFIX] Querier: Fix panic when marshaling QueryResultRequest. #6601
 * [BUGFIX] Ingester: Avoid resharding for query when restart readonly ingesters. #6642
 * [BUGFIX] Query Frontend: Fix query frontend per `user` metrics clean up. #6698
+* [BUGFIX] Add `__markers__` tenant ID validation. #6761
 
 ## 1.19.0 2025-02-27
 
diff --git a/docs/guides/limitations.md b/docs/guides/limitations.md
index 357f4e526ad..d8059b0b7c1 100644
--- a/docs/guides/limitations.md
+++ b/docs/guides/limitations.md
@@ -32,6 +32,13 @@ The following character sets are generally **safe for use in the tenant ID**:
 
 All other characters are not safe to use. In particular, slashes `/` and whitespaces (` `) are **not supported**.
 
+### Invalid tenant IDs
+The following tenant IDs are considered invalid in Cortex.
+
+- Current directory (`.`)
+- Parent directory (`..`)
+- Markers directory (`__markers__`)
+
 ### Length
 
 The tenant ID length should not exceed 150 bytes/characters.
diff --git a/pkg/storage/tsdb/tenant_deletion_mark.go b/pkg/storage/tsdb/tenant_deletion_mark.go
index 33ba7b22ceb..5e0eda2d34e 100644
--- a/pkg/storage/tsdb/tenant_deletion_mark.go
+++ b/pkg/storage/tsdb/tenant_deletion_mark.go
@@ -11,7 +11,7 @@ import (
 	"github.com/pkg/errors"
 	"github.com/thanos-io/objstore"
 
-	"github.com/cortexproject/cortex/pkg/util"
+	"github.com/cortexproject/cortex/pkg/tenant"
 	util_log "github.com/cortexproject/cortex/pkg/util/log"
 )
 
@@ -77,7 +77,7 @@ func GetLocalDeletionMarkPath(userID string) string {
 }
 
 func GetGlobalDeletionMarkPath(userID string) string {
-	return path.Join(util.GlobalMarkersDir, userID, TenantDeletionMarkFile)
+	return path.Join(tenant.GlobalMarkersDir, userID, TenantDeletionMarkFile)
 }
 
 func exists(ctx context.Context, bkt objstore.BucketReader, markerFile string) (bool, error) {
diff --git a/pkg/storage/tsdb/users_scanner.go b/pkg/storage/tsdb/users_scanner.go
index 0fe24395bf5..0e4f7acbe0b 100644
--- a/pkg/storage/tsdb/users_scanner.go
+++ b/pkg/storage/tsdb/users_scanner.go
@@ -8,13 +8,13 @@ import (
 	"github.com/go-kit/log/level"
 	"github.com/thanos-io/objstore"
 
-	"github.com/cortexproject/cortex/pkg/util"
+	"github.com/cortexproject/cortex/pkg/tenant"
 )
 
 // AllUsers returns true to each call and should be used whenever the UsersScanner should not filter out
 // any user due to sharding.
 func AllUsers(user string) (bool, error) {
-	if user == util.GlobalMarkersDir {
+	if user == tenant.GlobalMarkersDir {
 		return false, nil
 	}
 	return true, nil
@@ -52,7 +52,7 @@ func (s *UsersScanner) ScanUsers(ctx context.Context) (users, markedForDeletion
 	}
 
 	// Scan users from the __markers__ directory.
-	err = s.bucketClient.Iter(ctx, util.GlobalMarkersDir, func(entry string) error {
+	err = s.bucketClient.Iter(ctx, tenant.GlobalMarkersDir, func(entry string) error {
 		// entry will be of the form __markers__/<user>/
 		parts := strings.Split(entry, objstore.DirDelim)
 		userID := parts[1]
diff --git a/pkg/tenant/resolver_test.go b/pkg/tenant/resolver_test.go
index 6d52cc7369d..e29bde51b31 100644
--- a/pkg/tenant/resolver_test.go
+++ b/pkg/tenant/resolver_test.go
@@ -76,6 +76,12 @@ var commonResolverTestCases = []resolverTestCase{
 		errTenantID:  errTenantIDUnsafe,
 		errTenantIDs: errTenantIDUnsafe,
 	},
+	{
+		name:         "__markers__",
+		headerValue:  strptr("__markers__"),
+		errTenantID:  errTenantIDMarkers,
+		errTenantIDs: errTenantIDMarkers,
+	},
 	{
 		name:         "white space",
 		headerValue:  strptr(" "),
diff --git a/pkg/tenant/tenant.go b/pkg/tenant/tenant.go
index 57281239f62..07e8156d54c 100644
--- a/pkg/tenant/tenant.go
+++ b/pkg/tenant/tenant.go
@@ -10,9 +10,12 @@ import (
 	"github.com/weaveworks/common/user"
 )
 
+const GlobalMarkersDir = "__markers__"
+
 var (
 	errTenantIDTooLong = errors.New("tenant ID is too long: max 150 characters")
 	errTenantIDUnsafe  = errors.New("tenant ID is '.' or '..'")
+	errTenantIDMarkers = errors.New("tenant ID '__markers__' is not allowed")
 )
 
 type errTenantIDUnsupportedCharacter struct {
@@ -66,6 +69,12 @@ func ValidTenantID(s string) error {
 		return errTenantIDTooLong
 	}
 
+	// check tenantID is "__markers__"
+	if s == GlobalMarkersDir {
+		return errTenantIDMarkers
+	}
+
+	// check tenantID is "." or ".."
 	if containsUnsafePathSegments(s) {
 		return errTenantIDUnsafe
 	}
diff --git a/pkg/tenant/tenant_test.go b/pkg/tenant/tenant_test.go
index bc3a60b54fd..665f699c493 100644
--- a/pkg/tenant/tenant_test.go
+++ b/pkg/tenant/tenant_test.go
@@ -37,6 +37,10 @@ func TestValidTenantIDs(t *testing.T) {
 			name: "..",
 			err:  strptr("tenant ID is '.' or '..'"),
 		},
+		{
+			name: "__markers__",
+			err:  strptr("tenant ID '__markers__' is not allowed"),
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			err := ValidTenantID(tc.name)
diff --git a/pkg/util/allowed_tenants.go b/pkg/util/allowed_tenants.go
index d5b7af773e5..ff406b381b8 100644
--- a/pkg/util/allowed_tenants.go
+++ b/pkg/util/allowed_tenants.go
@@ -1,6 +1,6 @@
 package util
 
-const GlobalMarkersDir = "__markers__"
+import "github.com/cortexproject/cortex/pkg/tenant"
 
 // AllowedTenants that can answer whether tenant is allowed or not based on configuration.
 // Default value (nil) allows all tenants.
@@ -36,7 +36,7 @@ func NewAllowedTenants(enabled []string, disabled []string) *AllowedTenants {
 }
 
 func (a *AllowedTenants) IsAllowed(tenantID string) bool {
-	if tenantID == GlobalMarkersDir {
+	if tenantID == tenant.GlobalMarkersDir {
 		// __markers__ is reserved for global markers and no tenant should be allowed to have that name.
 		return false
 	}