diff --git a/moto/iam/policy_validation.py b/moto/iam/policy_validation.py
index c77069e70..8af17a7d2 100644
--- a/moto/iam/policy_validation.py
+++ b/moto/iam/policy_validation.py
@@ -568,4 +568,5 @@ class IAMTrustPolicyDocumentValidator(BaseIAMPolicyValidator):
             "sts:AssumeRole",
             "sts:AssumeRoleWithSAML",
             "sts:AssumeRoleWithWebIdentity",
+            "sts:TagSession",
         )