A future release of terraform should support using other s3 compatible backends like Spaces but none of the currently available versions(v0.10.7 is the current latest) will work.

I was able to configure terraform to store remote state in Spaces by building terraform from source and using the config

terraform {
  backend "s3" {
    bucket = "tfstate-bucket"
    key    = "path/terraform.tfstate"
    region = "us-east-1"
    endpoint = "https://nyc3.digitaloceanspaces.com"
    access_key = "redacted"
    secret_key = "redacted"
    skip_credentials_validation = true
    skip_get_ec2_platforms = true
    skip_requesting_account_id = true
    skip_metadata_api_check = true
  }
}

Note the region key is set to a known s3 region. Attempting to use an unknown region still causes terraform to complain

https://github.com/hashicorp/terraform/pull/15553

Works great for me too :) this is the config I used :

terraform {
        required_version = ">= 0.11, < 0.12"
        backend "s3" {
                skip_requesting_account_id = true
                skip_credentials_validation = true
                skip_get_ec2_platforms = true
                skip_metadata_api_check = true
                access_key = "XXXXXXXXX"
                secret_key = "XXXXXXXXXXXXXXXXXXXXXXXXX"
                endpoint = "https://xxx.digitaloceanspaces.com"
                region = "us-east-1"
                bucket = "XXXXXXX" // name of your space
                key = "production/terraform.tfstate"
        }
}

• endpoint should be in the form of 'https://ams3.digitaloceanspaces.com' ( example for when using ams3 as a region )
• bucket is the name of your space on digitalocean
• access_key is the key you'll find on your api page for spaces (https://cloud.digitalocean.com/settings/api/tokens)
• secret_key is the secret for the key ( not shown on the api page, but you can regenerate )
• key is the path on your space ( folder structure ) and the file it should save to