diff --git a/documentation/dsls/DSL:-Ash.Resource.md b/documentation/dsls/DSL:-Ash.Resource.md
index e1bf353a..6ebe07f6 100644
--- a/documentation/dsls/DSL:-Ash.Resource.md
+++ b/documentation/dsls/DSL:-Ash.Resource.md
@@ -581,9 +581,9 @@ belongs_to :word, Word, primary_key?: true, allow_nil?: false
 
 | Name | Type | Default | Docs |
 |------|------|---------|------|
-| [`through`](#relationships-many_to_many-through){: #relationships-many_to_many-through .spark-required} | `module` |  | The resource to use as the join resource. |
 | [`source_attribute_on_join_resource`](#relationships-many_to_many-source_attribute_on_join_resource){: #relationships-many_to_many-source_attribute_on_join_resource } | `atom` |  | The attribute on the join resource that should line up with `source_attribute` on this resource. Defaults to `<snake_cased_last_part_of_source_module_name>_id`. |
 | [`destination_attribute_on_join_resource`](#relationships-many_to_many-destination_attribute_on_join_resource){: #relationships-many_to_many-destination_attribute_on_join_resource } | `atom` |  | The attribute on the join resource that should line up with `destination_attribute` on the related resource. Defaults to `<snake_cased_last_part_of_destination_module_name>_id`. |
+| [`through`](#relationships-many_to_many-through){: #relationships-many_to_many-through } | `module` |  | The resource to use as the join resource. |
 | [`join_relationship`](#relationships-many_to_many-join_relationship){: #relationships-many_to_many-join_relationship } | `atom` |  | The has_many relationship to the join resource. Defaults to `<relationship_name>_join_assoc`. |
 | [`description`](#relationships-many_to_many-description){: #relationships-many_to_many-description } | `String.t` |  | An optional description for the relationship |
 | [`destination_attribute`](#relationships-many_to_many-destination_attribute){: #relationships-many_to_many-destination_attribute } | `atom` | `:id` | The attribute on the related resource that should match the `source_attribute` configured on this resource. |
diff --git a/lib/ash/resource/relationships/many_to_many.ex b/lib/ash/resource/relationships/many_to_many.ex
index 4c1ae305..25fd195f 100644
--- a/lib/ash/resource/relationships/many_to_many.ex
+++ b/lib/ash/resource/relationships/many_to_many.ex
@@ -58,19 +58,16 @@ defmodule Ash.Resource.Relationships.ManyToMany do
                 [
                   source_attribute_on_join_resource: [
                     type: :atom,
-                    required: false,
                     doc:
                       "The attribute on the join resource that should line up with `source_attribute` on this resource. Defaults to `<snake_cased_last_part_of_source_module_name>_id`."
                   ],
                   destination_attribute_on_join_resource: [
                     type: :atom,
-                    required: false,
                     doc:
                       "The attribute on the join resource that should line up with `destination_attribute` on the related resource. Defaults to `<snake_cased_last_part_of_destination_module_name>_id`."
                   ],
                   through: [
                     type: Ash.OptionsHelpers.ash_resource(),
-                    required: true,
                     doc: "The resource to use as the join resource."
                   ],
                   join_relationship: [
diff --git a/lib/ash/resource/transformers/create_join_relationship.ex b/lib/ash/resource/transformers/create_join_relationship.ex
index fe1bc40c..2170d9a5 100644
--- a/lib/ash/resource/transformers/create_join_relationship.ex
+++ b/lib/ash/resource/transformers/create_join_relationship.ex
@@ -4,7 +4,7 @@ defmodule Ash.Resource.Transformers.CreateJoinRelationship do
   """
   use Spark.Dsl.Transformer
 
-  alias Spark.Dsl.Transformer
+  alias Spark.{Dsl.Transformer, Error.DslError}
 
   @extension Ash.Resource.Dsl
 
@@ -12,38 +12,63 @@ defmodule Ash.Resource.Transformers.CreateJoinRelationship do
     dsl_state
     |> Transformer.get_entities([:relationships])
     |> Enum.filter(&(&1.type == :many_to_many))
-    |> Enum.reject(fn relationship ->
+    |> Enum.reduce_while({:ok, dsl_state}, fn relationship, {:ok, dsl_state} ->
       dsl_state
       |> Transformer.get_entities([:relationships])
       |> Enum.find(&(&1.name == relationship.join_relationship))
-    end)
-    |> Enum.reduce({:ok, dsl_state}, fn relationship, {:ok, dsl_state} ->
-      autogenerated_join_relationship_of = relationship.name
+      |> case do
+        nil when relationship.through == nil ->
+          error =
+            DslError.exception(
+              path: [:relationships, relationship.name],
+              message:
+                "Either `through` or `join_relationship` with an existing relationship is required."
+            )
 
-      {:ok, relationship} =
-        Transformer.build_entity(
-          @extension,
-          [:relationships],
-          :has_many,
-          [
-            name: relationship.join_relationship,
-            destination: relationship.through,
-            destination_attribute: relationship.source_attribute_on_join_resource,
-            api: relationship.api,
-            source_attribute: relationship.source_attribute,
-            private?: true
-          ]
-          |> add_messages(relationship)
-        )
+          {:halt, {:error, error}}
 
-      relationship =
-        Map.put(
-          relationship,
-          :autogenerated_join_relationship_of,
-          autogenerated_join_relationship_of
-        )
+        nil ->
+          {:ok, join_relationship} =
+            Transformer.build_entity(
+              @extension,
+              [:relationships],
+              :has_many,
+              [
+                name: relationship.join_relationship,
+                destination: relationship.through,
+                destination_attribute: relationship.source_attribute_on_join_resource,
+                api: relationship.api,
+                source_attribute: relationship.source_attribute,
+                private?: true
+              ]
+              |> add_messages(relationship)
+            )
 
-      {:ok, Transformer.add_entity(dsl_state, [:relationships], relationship)}
+          join_relationship =
+            Map.put(join_relationship, :autogenerated_join_relationship_of, relationship.name)
+
+          dsl_state = Transformer.add_entity(dsl_state, [:relationships], join_relationship)
+
+          {:cont, {:ok, dsl_state}}
+
+        join_relationship ->
+          relationship =
+            %{
+              relationship
+              | through: join_relationship.destination,
+                destination_attribute_on_join_resource: join_relationship.destination_attribute
+            }
+
+          dsl_state =
+            Transformer.replace_entity(
+              dsl_state,
+              [:relationships],
+              relationship,
+              &(&1.name == relationship.name)
+            )
+
+          {:cont, {:ok, dsl_state}}
+      end
     end)
   end