diff --git a/modules/cloudsql-instance/main.tf b/modules/cloudsql-instance/main.tf
index e64ee383b..b5492bc8b 100644
--- a/modules/cloudsql-instance/main.tf
+++ b/modules/cloudsql-instance/main.tf
@@ -215,7 +215,7 @@ resource "google_sql_database_instance" "replicas" {
   master_instance_name = google_sql_database_instance.primary.name
 
   settings {
-    tier                        = var.tier
+    tier                        = coalesce(each.value.tier, var.tier)
     edition                     = var.edition
     deletion_protection_enabled = var.gcp_deletion_protection
     disk_autoresize             = var.disk_size == null
diff --git a/modules/cloudsql-instance/variables.tf b/modules/cloudsql-instance/variables.tf
index 87616b233..803d59cdb 100644
--- a/modules/cloudsql-instance/variables.tf
+++ b/modules/cloudsql-instance/variables.tf
@@ -239,10 +239,11 @@ variable "region" {
 }
 
 variable "replicas" {
-  description = "Map of NAME=> {REGION, KMS_KEY} for additional read replicas. Set to null to disable replica creation."
+  description = "Map of NAME=> {REGION, KMS_KEY, TIER} for additional read replicas. Tier defaults to the primary instance tier when unset. Set to null to disable replica creation."
   type = map(object({
     region              = string
     encryption_key_name = optional(string)
+    tier                = optional(string)
   }))
   default  = {}
   nullable = false
@@ -287,6 +288,7 @@ variable "terraform_deletion_protection" {
 variable "tier" {
   description = "The machine type to use for the instances."
   type        = string
+  nullable    = false
 }
 
 variable "time_zone" {