diff --git a/lib/ecto/repo/schema.ex b/lib/ecto/repo/schema.ex
index 730e50a933..4d32d2f95b 100644
--- a/lib/ecto/repo/schema.ex
+++ b/lib/ecto/repo/schema.ex
@@ -458,7 +458,7 @@ defmodule Ecto.Repo.Schema do
 
     wrap_in_transaction(adapter, adapter_meta, opts, changeset, assocs, embeds, prepare, fn ->
       assoc_opts = assoc_opts(assocs, opts)
-      user_changeset = run_prepare(changeset, prepare)
+      user_changeset = changeset |> run_prepare(prepare) |> drop_non_writable_changes!(drop_fields, schema, :insert)
 
       {changeset, parents, children, _} = pop_assocs(user_changeset, assocs)
       changeset = process_parents(changeset, user_changeset, parents, [], adapter, assoc_opts)
@@ -584,7 +584,7 @@ defmodule Ecto.Repo.Schema do
     if changeset.changes != %{} or force? do
       wrap_in_transaction(adapter, adapter_meta, opts, changeset, assocs, embeds, prepare, fn ->
         assoc_opts = assoc_opts(assocs, opts)
-        user_changeset = run_prepare(changeset, prepare)
+        user_changeset = changeset |> run_prepare(prepare) |> drop_non_writable_changes!(drop_fields, schema, :update)
 
         {changeset, parents, children, reset_parents} = pop_assocs(user_changeset, assocs)
 
diff --git a/test/ecto/repo_test.exs b/test/ecto/repo_test.exs
index 5d9f34587b..97c01bfe4b 100644
--- a/test/ecto/repo_test.exs
+++ b/test/ecto/repo_test.exs
@@ -2521,6 +2521,28 @@ defmodule Ecto.RepoTest do
       assert_received {:update, %{changes: [always: 12]}}
     end
 
+    test "update enforces writable fields added by prepare_changes" do
+      %{always: 10, never: nil} =
+        %MySchemaWritable{id: 1}
+        |> Ecto.Changeset.change(%{always: 10})
+        |> Ecto.Changeset.prepare_changes(&Ecto.Changeset.put_change(&1, :never, 11))
+        |> TestRepo.update!()
+
+      assert_received {:update, %{changes: [always: 10]}}
+
+      message = ~r"""
+      you are attempting to write to the field :never of #{inspect(__MODULE__.MySchemaWritableRaise)} but
+      the `:writable` option of this field indicates the field should not be written to during an update.
+      """
+
+      assert_raise ArgumentError, message, fn ->
+        %MySchemaWritableRaise{id: 2}
+        |> Ecto.Changeset.change(%{always: 12})
+        |> Ecto.Changeset.prepare_changes(&Ecto.Changeset.put_change(&1, :never, 13))
+        |> TestRepo.update!()
+      end
+    end
+
     test "update is a no-op when updatable fields are not changed" do
       %MySchemaWritable{id: 1}
       |> Ecto.Changeset.change(%{never: "can't update", insert: "can't update either"})
@@ -2654,6 +2676,29 @@ defmodule Ecto.RepoTest do
       assert Enum.sort(inserted_fields) == [always: 12, id: 2, insert: 11]
     end
 
+    test "insert enforces writable fields added by prepare_changes" do
+      %{always: 10, never: nil} =
+        %MySchemaWritable{id: 1}
+        |> Ecto.Changeset.change(%{always: 10})
+        |> Ecto.Changeset.prepare_changes(&Ecto.Changeset.put_change(&1, :never, 11))
+        |> TestRepo.insert!()
+
+      assert_received {:insert, %{fields: inserted_fields}}
+      assert Enum.sort(inserted_fields) == [always: 10, id: 1]
+
+      message = ~r"""
+      you are attempting to write to the field :never of #{inspect(__MODULE__.MySchemaWritableRaise)} but
+      the `:writable` option of this field indicates the field should not be written to during an insert.
+      """
+
+      assert_raise ArgumentError, message, fn ->
+        %MySchemaWritableRaise{id: 2}
+        |> Ecto.Changeset.change(%{always: 12})
+        |> Ecto.Changeset.prepare_changes(&Ecto.Changeset.put_change(&1, :never, 13))
+        |> TestRepo.insert!()
+      end
+    end
+
     test "insert with returning" do
       %MySchemaWritable{id: 1}
       |> Ecto.Changeset.change(%{always: 10, never: 11, insert: 12})