diff --git a/README.md b/README.md
index fb390fb9..963397d9 100644
--- a/README.md
+++ b/README.md
@@ -133,7 +133,6 @@ Under-the-hood `JSONAPI.EnsureSpec` relies on three individual plugs:
 config :jsonapi,
   host: "www.someotherhost.com",
   scheme: "https",
-  underscore_to_dash: true, # DEPRECATED
   field_transformation: :underscore,
   remove_links: false,
   json_library: Jason
@@ -151,9 +150,6 @@ config :jsonapi,
   `"favorite-color": blue`). If your API uses dashed fields, set this value to
   `:dasherize`. If your API uses underscores (e.g. `"favorite_color": "red"`)
   set to `:underscore`.
-- **underscore_to_dash**. This is a deprecated option that previously defaulted
-  to `false`. Please set the appropriate value on the `field_transformation`
-  configuration option.
 
 ## Other
 
diff --git a/lib/jsonapi/deprecation.ex b/lib/jsonapi/deprecation.ex
index ed769803..4d8b6d28 100644
--- a/lib/jsonapi/deprecation.ex
+++ b/lib/jsonapi/deprecation.ex
@@ -27,11 +27,4 @@ defmodule JSONAPI.Deprecation do
       Macro.Env.stacktrace(__ENV__)
     )
   end
-
-  def warn(:underscore_to_dash) do
-    IO.warn(
-      "`:underscore_to_dash` is deprecated. If you want underscored fields, set `:field_transformation` to :underscore. If you want your fields to be dashed, set to :dasherized",
-      Macro.Env.stacktrace(__ENV__)
-    )
-  end
 end
diff --git a/lib/jsonapi/utils/string.ex b/lib/jsonapi/utils/string.ex
index 55ade066..0a1ec247 100644
--- a/lib/jsonapi/utils/string.ex
+++ b/lib/jsonapi/utils/string.ex
@@ -179,18 +179,6 @@ defmodule JSONAPI.Utils.String do
     fun.(value)
   end
 
-  defp normalized_underscore_to_dash_config(value) when is_boolean(value) do
-    Deprecation.warn(:underscore_to_dash)
-
-    if value do
-      :dasherize
-    else
-      :underscore
-    end
-  end
-
-  defp normalized_underscore_to_dash_config(value) when is_nil(value), do: value
-
   @doc """
   The configured transformation for the API's fields. JSON:API v1.1 recommends
   using camlized fields (e.g. "goodDog", versus "good_dog").  However, we don't hold a strong
@@ -219,8 +207,7 @@ defmodule JSONAPI.Utils.String do
   ```
   """
   def field_transformation do
-    normalized_underscore_to_dash_config(Application.get_env(:jsonapi, :underscore_to_dash)) ||
-      field_transformation(Application.get_env(:jsonapi, :field_transformation))
+    field_transformation(Application.get_env(:jsonapi, :field_transformation))
   end
 
   @doc false
diff --git a/test/utils/string_test.exs b/test/utils/string_test.exs
index 512f5fea..3f913a9a 100644
--- a/test/utils/string_test.exs
+++ b/test/utils/string_test.exs
@@ -6,36 +6,4 @@ defmodule JSONAPI.Utils.StringTest do
   import JSONAPI.Utils.String
 
   doctest JSONAPI.Utils.String
-
-  describe "legacy configuration to dasherize fields" do
-    setup do
-      Application.put_env(:jsonapi, :underscore_to_dash, true)
-
-      on_exit(fn ->
-        Application.delete_env(:jsonapi, :underscore_to_dash)
-      end)
-
-      {:ok, []}
-    end
-
-    test "#field_transformation/0 returns :dasherize" do
-      assert field_transformation() == :dasherize
-    end
-  end
-
-  describe "legacy configuration to underscore fields" do
-    setup do
-      Application.put_env(:jsonapi, :underscore_to_dash, false)
-
-      on_exit(fn ->
-        Application.delete_env(:jsonapi, :underscore_to_dash)
-      end)
-
-      {:ok, []}
-    end
-
-    test "#field_transformation/0 returns :underscore" do
-      assert field_transformation() == :underscore
-    end
-  end
 end