diff --git a/spec/04-services/05-sts_spec.lua b/spec/04-services/05-sts_spec.lua
new file mode 100644
index 0000000..cb7660e
--- /dev/null
+++ b/spec/04-services/05-sts_spec.lua
@@ -0,0 +1,188 @@
+setmetatable(_G, nil)
+
+-- -- hock request sending
+-- package.loaded["resty.aws.request.execute"] = function(...)
+--   return ...
+-- end
+
+local AWS = require("resty.aws")
+local AWS_global_config = require("resty.aws.config").global
+
+local config = AWS_global_config
+local aws = AWS(config)
+
+aws.config.credentials = aws:Credentials {
+  accessKeyId = "test_id",
+  secretAccessKey = "test_key",
+}
+
+-- aws.config.region = "test_region"
+
+local test_assume_role_arn = "arn:aws:iam::123456789012:role/test-role"
+local test_role_session_name = "lua-resty-aws-test-assumeRole"
+
+describe("STS service", function()
+  local origin_time
+  setup(function()
+    origin_time = ngx.time
+    ngx.time = function () --luacheck: ignore
+      return 1667543171
+    end
+  end)
+
+  teardown(function ()
+    ngx.time = origin_time --luacheck: ignore
+  end)
+
+  -- before_each(function()
+  --   sts = aws:STS()
+  -- end)
+
+  -- after_each(function()
+
+  -- end)
+
+  for _, region in ipairs({"us-east-1", "us-east-2", "ap-south-1", "ca-west-1", "eu-west-2", "sa-east-1"}) do
+    describe("In Region #" .. region, function ()
+      -- before_each(function()
+      --   aws.config.region = region
+      -- end)
+
+      it("AWS_STS_REGIONAL_ENDPOINT==regional with default endpoint", function ()
+        local config = {
+          region = region,
+          stsRegionalEndpoints = "regional",
+          dry_run = true,
+        }
+
+        local sts = aws:STS(config)
+        local request = sts:assumeRole({
+          RoleArn = test_assume_role_arn,
+          RoleSessionName = test_role_session_name,
+        })
+
+        assert.same(sts.config.stsRegionalEndpoints, "regional")
+        -- Check the signing region has been injected
+        assert.same(region, sts.config.signingRegion)
+        assert.truthy(sts.config._regionalEndpointInjected)
+        -- Check the endpoint has been injected
+        assert.same(sts.config.endpoint, "https://sts." .. region .. ".amazonaws.com")
+        assert.not_nil(request.headers.Authorization:find(region, 1, true))
+      end)
+
+      describe("AWS_STS_REGIONAL_ENDPOINT==regional with non-default endpoint", function()
+        it("and endpoint is regional domain", function ()
+          local config = {
+            region = region,
+            stsRegionalEndpoints = "regional",
+            endpoint = "https://sts." .. region .. ".amazonaws.com",
+            dry_run = true,
+          }
+
+          local sts = aws:STS(config)
+          local request = sts:assumeRole({
+            RoleArn = test_assume_role_arn,
+            RoleSessionName = test_role_session_name,
+          })
+
+          assert.same(sts.config.stsRegionalEndpoints, "regional")
+          -- Check the signing region has been injected
+          assert.same(region, sts.config.signingRegion)
+          assert.truthy(sts.config._regionalEndpointInjected)
+          -- Check thes endpoint has not been injected twice
+          assert.same(sts.config.endpoint, config.endpoint)
+          assert.not_nil(request.headers.Authorization:find(region, 1, true))
+        end)
+
+        it("and endpoint is global domain", function ()
+          local config = {
+            region = region,
+            stsRegionalEndpoints = "regional",
+            endpoint = "https://sts.amazonaws.com",
+            dry_run = true,
+          }
+
+          local sts = aws:STS(config)
+          local request = sts:assumeRole({
+            RoleArn = test_assume_role_arn,
+            RoleSessionName = test_role_session_name,
+          })
+
+          assert.same(sts.config.stsRegionalEndpoints, "regional")
+          -- Check the signing region has been injected
+          assert.same(region, sts.config.signingRegion)
+          assert.truthy(sts.config._regionalEndpointInjected)
+          -- Check the endpoint has been injected
+          assert.same(sts.config.endpoint, "https://sts." .. region .. ".amazonaws.com")
+          assert.not_nil(request.headers.Authorization:find(region, 1, true))
+        end)
+
+        it("and endpoint is region VPC endpoint", function ()
+          local config = {
+            region = region,
+            stsRegionalEndpoints = "regional",
+            endpoint = "https://vpce-1234567-abcdefg.sts." .. region .. ".vpce.amazonaws.com",
+            dry_run = true,
+          }
+
+          local sts = aws:STS(config)
+          local request = sts:assumeRole({
+            RoleArn = test_assume_role_arn,
+            RoleSessionName = test_role_session_name,
+          })
+
+          assert.same(sts.config.stsRegionalEndpoints, "regional")
+          -- Check the signing region has been injected
+          assert.same(region, sts.config.signingRegion)
+          assert.truthy(sts.config._regionalEndpointInjected)
+          -- Check the endpoint has not been injected when endpoint is a vpc endpoint
+          assert.same(sts.config.endpoint, config.endpoint)
+          assert.not_nil(request.headers.Authorization:find(region, 1, true))
+        end)
+
+        it("and endpoint is AZ VPC endpoint", function ()
+          local config = {
+            region = region,
+            stsRegionalEndpoints = "regional",
+            endpoint = "https://vpce-1234567-abcdefg-" .. region .. "c" .. ".sts." .. region .. ".vpce.amazonaws.com",
+            dry_run = true,
+          }
+
+          local sts = aws:STS(config)
+          local request = sts:assumeRole({
+            RoleArn = test_assume_role_arn,
+            RoleSessionName = test_role_session_name,
+          })
+
+          assert.same(sts.config.stsRegionalEndpoints, "regional")
+          -- Check the signing region has been injected
+          assert.same(region, sts.config.signingRegion)
+          assert.truthy(sts.config._regionalEndpointInjected)
+          -- Check the endpoint has not been injected when endpoint is a vpc endpoint
+          assert.same(sts.config.endpoint, config.endpoint)
+          assert.not_nil(request.headers.Authorization:find(region, 1, true))
+        end)
+      end)
+
+      it("AWS_STS_REGIONAL_ENDPOINT==legacy with default endpoint", function ()
+        local config = {
+          region = region,
+          stsRegionalEndpoints = "legacy",
+          dry_run = true,
+        }
+
+        local sts = aws:STS(config)
+        local request = sts:assumeRole({
+          RoleArn = test_assume_role_arn,
+          RoleSessionName = test_role_session_name,
+        })
+
+        assert.same(sts.config.stsRegionalEndpoints, "legacy")
+        assert.same("us-east-1", sts.config.signingRegion)
+        assert.is_nil(sts.config._regionalEndpointInjected)
+        assert.same(sts.config.endpoint, "https://sts.amazonaws.com")
+        assert.not_nil(request.headers.Authorization:find("us-east-1", 1, true))
+      end)
+    end)
+  end
+end)
diff --git a/src/resty/aws/init.lua b/src/resty/aws/init.lua
index f5b7b2a..9d3b9a1 100644
--- a/src/resty/aws/init.lua
+++ b/src/resty/aws/init.lua
@@ -260,6 +260,24 @@ do
 end
 
 
+local is_regional_sts_domain do
+  -- from the list described in https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_temp_enable-regions.html
+  -- TODO: not sure if gov cloud also has their own endpoints so leave it for now
+  local stsRegionRegexes = {
+    [[sts\.(us|eu|ap|sa|ca|me)\-\w+\-\d+\.amazonaws\.com$]],
+    [[sts\.cn\-\w+\-\d+\.amazonaws\.com\.cn$]],
+  }
+
+  function is_regional_sts_domain(domain)
+    for _, entry in ipairs(stsRegionRegexes) do
+      if ngx.re.match(domain, entry, "jo") then
+        return true
+      end
+    end
+
+    return false
+  end
+end
 
 -- written from scratch
 
@@ -325,14 +343,19 @@ local function generate_service_methods(service)
         -- https://github.com/aws/aws-sdk-js/blob/307e82673b48577fce4389e4ce03f95064e8fe0d/lib/services/sts.js#L78-L82
         assert(service.config.region, "region is required when using STS regional endpoints")
 
-        -- If the endpoint is a VPC endpoint DNS hostname then we don't need to inject the region
-        -- VPC endpoint DNS hostnames always contain region, see
-        -- https://docs.aws.amazon.com/vpc/latest/privatelink/privatelink-access-aws-services.html#interface-endpoint-dns-hostnames
-        if not service.config._regionalEndpointInjected and not service.config.endpoint:match(AWS_VPC_ENDPOINT_DOMAIN_PATTERN) then
-          local pre, post = service.config.endpoint:match(AWS_PUBLIC_DOMAIN_PATTERN)
-          service.config.endpoint = pre .. "." .. service.config.region .. post
-          service.config.signingRegion = service.config.region
+        if not service.config._regionalEndpointInjected then
           service.config._regionalEndpointInjected = true
+          -- stsRegionalEndpoints is set to 'regional', so inject region into the
+          -- signingRegion to override global region_config_data
+          service.config.signingRegion = service.config.region
+
+          -- If the endpoint is a VPC endpoint DNS hostname, or a regional STS domain, then we don't need to inject the region
+          -- VPC endpoint DNS hostnames always contain region, see
+          -- https://docs.aws.amazon.com/vpc/latest/privatelink/privatelink-access-aws-services.html#interface-endpoint-dns-hostnames
+          if not service.config.endpoint:match(AWS_VPC_ENDPOINT_DOMAIN_PATTERN) and not is_regional_sts_domain(service.config.endpoint) then
+            local pre, post = service.config.endpoint:match(AWS_PUBLIC_DOMAIN_PATTERN)
+            service.config.endpoint = pre .. "." .. service.config.region .. post
+          end
         end
       end