diff --git a/lib/ecto/adapters/postgres/connection.ex b/lib/ecto/adapters/postgres/connection.ex
index 3248f144..df9ccbaf 100644
--- a/lib/ecto/adapters/postgres/connection.ex
+++ b/lib/ecto/adapters/postgres/connection.ex
@@ -1602,10 +1602,11 @@ if Code.ensure_loaded?(Postgrex) do
         "ALTER COLUMN ",
         quote_name(name),
         " TYPE ",
-        column_type(type, opts),
+        modify_column_type(type, opts),
         modify_null(name, opts),
         modify_default(name, type, opts),
-        collation_expr(collation)
+        collation_expr(collation),
+        modify_identity(name, type, opts)
       ]
     end
 
@@ -1632,6 +1633,19 @@ if Code.ensure_loaded?(Postgrex) do
     defp column_change(_table, {:remove_if_exists, name}),
       do: ["DROP COLUMN IF EXISTS ", quote_name(name)]
 
+    # `:identity`'s "GENERATED BY DEFAULT AS IDENTITY" is valid as part of a column
+    # definition (CREATE TABLE/ADD COLUMN), but PostgreSQL rejects it combined with
+    # ALTER COLUMN ... TYPE - it must be added via a separate ALTER COLUMN ... ADD
+    # GENERATED clause (see modify_identity/3 below).
+    defp modify_column_type(:identity, opts), do: column_type_name(:identity, opts)
+    defp modify_column_type(type, opts), do: column_type(type, opts)
+
+    defp modify_identity(name, :identity, opts) do
+      [", ALTER COLUMN ", quote_name(name), " ADD", identity_generated_expr(opts)]
+    end
+
+    defp modify_identity(_name, _type, _opts), do: []
+
     defp modify_null(name, opts) do
       case Keyword.get(opts, :null) do
         true -> [", ALTER COLUMN ", quote_name(name), " DROP NOT NULL"]
@@ -1798,22 +1812,7 @@ if Code.ensure_loaded?(Postgrex) do
 
       case Keyword.get(opts, :generated) do
         nil when type == :identity ->
-          cleanup = fn v -> is_integer(v) and v > 0 end
-
-          sequence =
-            [Keyword.get(opts, :start_value)]
-            |> Enum.filter(cleanup)
-            |> Enum.map(&"START WITH #{&1}")
-            |> Kernel.++(
-              [Keyword.get(opts, :increment)]
-              |> Enum.filter(cleanup)
-              |> Enum.map(&"INCREMENT BY #{&1}")
-            )
-
-          case sequence do
-            [] -> [type_name, " GENERATED BY DEFAULT AS IDENTITY"]
-            _ -> [type_name, " GENERATED BY DEFAULT AS IDENTITY(", Enum.join(sequence, " "), ") "]
-          end
+          [type_name, identity_generated_expr(opts)]
 
         nil ->
           type_name
@@ -1827,6 +1826,25 @@ if Code.ensure_loaded?(Postgrex) do
       end
     end
 
+    defp identity_generated_expr(opts) do
+      cleanup = fn v -> is_integer(v) and v > 0 end
+
+      sequence =
+        [Keyword.get(opts, :start_value)]
+        |> Enum.filter(cleanup)
+        |> Enum.map(&"START WITH #{&1}")
+        |> Kernel.++(
+          [Keyword.get(opts, :increment)]
+          |> Enum.filter(cleanup)
+          |> Enum.map(&"INCREMENT BY #{&1}")
+        )
+
+      case sequence do
+        [] -> " GENERATED BY DEFAULT AS IDENTITY"
+        _ -> [" GENERATED BY DEFAULT AS IDENTITY(", Enum.join(sequence, " "), ") "]
+      end
+    end
+
     defp column_type_name({:array, type}, opts) do
       [column_type_name(type, opts), "[]"]
     end
diff --git a/test/ecto/adapters/postgres_test.exs b/test/ecto/adapters/postgres_test.exs
index e907da5e..25788087 100644
--- a/test/ecto/adapters/postgres_test.exs
+++ b/test/ecto/adapters/postgres_test.exs
@@ -2391,6 +2391,35 @@ defmodule Ecto.Adapters.PostgresTest do
            ]
   end
 
+  test "alter table modify to identity" do
+    alter =
+      {:alter, table(:posts),
+       [
+         {:modify, :field, :identity, []}
+       ]}
+
+    assert execute_ddl(alter) == [
+             """
+             ALTER TABLE "posts"
+             ALTER COLUMN "field" TYPE bigint,
+             ALTER COLUMN "field" ADD GENERATED BY DEFAULT AS IDENTITY
+             """
+             |> remove_newlines
+           ]
+  end
+
+  test "alter table modify to identity with options" do
+    alter =
+      {:alter, table(:posts),
+       [
+         {:modify, :field, :identity, [start_value: 1_000, increment: 10]}
+       ]}
+
+    assert execute_ddl(alter) == [
+             ~s|ALTER TABLE "posts" ALTER COLUMN "field" TYPE bigint, ALTER COLUMN "field" ADD GENERATED BY DEFAULT AS IDENTITY(START WITH 1000 INCREMENT BY 10) |
+           ]
+  end
+
   test "create table with generated column" do
     create =
       {:create, table(:posts),