diff --git a/modules/transit_gateway/README.md b/modules/transit_gateway/README.md
index f5002687..92720e48 100644
--- a/modules/transit_gateway/README.md
+++ b/modules/transit_gateway/README.md
@@ -54,6 +54,7 @@ No modules.
 | <a name="input_route_tables"></a> [route\_tables](#input\_route\_tables) | n/a | `map` | `{}` | no |
 | <a name="input_shared_principals"></a> [shared\_principals](#input\_shared\_principals) | n/a | `map` | `{}` | no |
 | <a name="input_tags"></a> [tags](#input\_tags) | Optional Map of arbitrary tags to apply to all resources | `map(string)` | `{}` | no |
+| <a name="input_transit_gateway_cidr_blocks"></a> [transit\_gateway\_cidr\_blocks](#input\_transit\_gateway\_cidr\_blocks) | One or more IPv4 or IPv6 CIDR blocks for the transit gateway. Must be a size /24 CIDR block or larger for IPv4, or a size /64 CIDR block or larger for IPv6. | `set(string)` | `[]` | no |
 | <a name="input_vpn_ecmp_support"></a> [vpn\_ecmp\_support](#input\_vpn\_ecmp\_support) | See the [provider documentation](https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/ec2_transit_gateway). | `string` | `null` | no |
 
 ### Outputs
diff --git a/modules/transit_gateway/main.tf b/modules/transit_gateway/main.tf
index 0531c11e..258f6e56 100644
--- a/modules/transit_gateway/main.tf
+++ b/modules/transit_gateway/main.tf
@@ -19,6 +19,7 @@ resource "aws_ec2_transit_gateway" "this" {
   auto_accept_shared_attachments  = var.auto_accept_shared_attachments
   default_route_table_association = "disable"
   default_route_table_propagation = "disable"
+  transit_gateway_cidr_blocks     = var.transit_gateway_cidr_blocks
   dns_support                     = var.dns_support
   vpn_ecmp_support                = var.vpn_ecmp_support
   tags                            = merge(var.tags, { Name = var.name })
diff --git a/modules/transit_gateway/variables.tf b/modules/transit_gateway/variables.tf
index b6481b8d..f41a925d 100644
--- a/modules/transit_gateway/variables.tf
+++ b/modules/transit_gateway/variables.tf
@@ -51,6 +51,16 @@ variable "shared_principals" {
   default = {}
 }
 
+variable "transit_gateway_cidr_blocks" {
+  description = "One or more IPv4 or IPv6 CIDR blocks for the transit gateway. Must be a size /24 CIDR block or larger for IPv4, or a size /64 CIDR block or larger for IPv6."
+  default     = []
+  type        = set(string)
+  validation {
+    condition     = alltrue([for cidr in var.transit_gateway_cidr_blocks : can(cidrsubnet(cidr, 0, 0))])
+    error_message = "Transit gateway CIDR blocks must contain valid IPv4 or IPv6 CIDR."
+  }
+}
+
 variable "tags" {
   description = "Optional Map of arbitrary tags to apply to all resources"
   type        = map(string)