diff --git a/website/docs/r/rds_cluster.html.markdown b/website/docs/r/rds_cluster.html.markdown
index 79a6302c1753..ae42ad6fe785 100644
--- a/website/docs/r/rds_cluster.html.markdown
+++ b/website/docs/r/rds_cluster.html.markdown
@@ -206,6 +206,7 @@ This resource supports the following arguments:
   RDS automatically assigns 3 AZs if less than 3 AZs are configured, which will show as a difference requiring resource recreation next Terraform apply.
   We recommend specifying 3 AZs or using [the `lifecycle` configuration block `ignore_changes` argument](https://www.terraform.io/docs/configuration/meta-arguments/lifecycle.html#ignore_changes) if necessary.
   A maximum of 3 AZs can be configured.
+  **Note:** [Multi-AZ DB clusters](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/multi-az-db-clusters-concepts.html) require exactly 3 Availability Zones in the DB subnet group. Aurora DB clusters can operate with fewer AZs, but RDS will still automatically assign 3 AZs as described above.
 * `backtrack_window` - (Optional) Target backtrack window, in seconds. Only available for `aurora` and `aurora-mysql` engines currently. To disable backtracking, set this value to `0`. Defaults to `0`. Must be between `0` and `259200` (72 hours)
 * `backup_retention_period` - (Optional) Days to retain backups for. Default `1`
 * `ca_certificate_identifier` - (Optional) The CA certificate identifier to use for the DB cluster's server certificate.