atlantis/runatlantis.io/docs/terraform-versions.md

Terraform Versions

You can customize which version of Terraform Atlantis defaults to by setting the --default-tf-version flag (ex. --default-tf-version=v0.12.0).

Via atlantis.yaml

If you wish to use a different version than the default for a specific repo or project, you need to create an atlantis.yaml file and set the terraform_version key:

version: 3
projects:
- dir: .
  terraform_version: v0.10.5

See atlantis.yaml Use Cases for more details.

Via terraform config

Alternatively, one can use the terraform configuration block's required_version key to specify an exact version:

terraform {
  required_version = "0.12.0"
}

See Terraform required_version for reference.

::: tip NOTE Atlantis will automatically download the version specified. :::

::: tip NOTE The Atlantis latest docker image tends to have recent versions of Terraform, but there may be a delay as new versions are released. The highest version of Terraform allowed in your code is the version specified by DEFAULT_TERRAFORM_VERSION in the image your server is running. :::