diff --git a/lib/ecto/adapters/myxql/connection.ex b/lib/ecto/adapters/myxql/connection.ex
index 833c6d35..e6feb8dc 100644
--- a/lib/ecto/adapters/myxql/connection.ex
+++ b/lib/ecto/adapters/myxql/connection.ex
@@ -941,6 +941,9 @@ if Code.ensure_loaded?(MyXQL) do
     defp op_to_binary({:is_nil, _, [_]} = expr, sources, query),
       do: paren_expr(expr, sources, query)
 
+    defp op_to_binary({:not, _, [_]} = expr, sources, query),
+      do: paren_expr(expr, sources, query)
+
     defp op_to_binary(expr, sources, query),
       do: expr(expr, sources, query)
 
diff --git a/lib/ecto/adapters/postgres/connection.ex b/lib/ecto/adapters/postgres/connection.ex
index b6398531..64cb4081 100644
--- a/lib/ecto/adapters/postgres/connection.ex
+++ b/lib/ecto/adapters/postgres/connection.ex
@@ -1192,6 +1192,9 @@ if Code.ensure_loaded?(Postgrex) do
     defp maybe_paren({:is_nil, _, [_]} = expr, sources, query),
       do: paren_expr(expr, sources, query)
 
+    defp maybe_paren({:not, _, [_]} = expr, sources, query),
+      do: paren_expr(expr, sources, query)
+
     defp maybe_paren(expr, sources, query),
       do: expr(expr, sources, query)
 
diff --git a/lib/ecto/adapters/tds/connection.ex b/lib/ecto/adapters/tds/connection.ex
index be1cf229..27405d4b 100644
--- a/lib/ecto/adapters/tds/connection.ex
+++ b/lib/ecto/adapters/tds/connection.ex
@@ -1009,6 +1009,10 @@ if Code.ensure_loaded?(Tds) do
       paren_expr(expr, sources, query)
     end
 
+    defp op_to_binary({:not, _, [_]} = expr, sources, query) do
+      paren_expr(expr, sources, query)
+    end
+
     defp op_to_binary(expr, sources, query) do
       expr(expr, sources, query)
     end
diff --git a/test/ecto/adapters/myxql_test.exs b/test/ecto/adapters/myxql_test.exs
index b062d7bb..8e8485b8 100644
--- a/test/ecto/adapters/myxql_test.exs
+++ b/test/ecto/adapters/myxql_test.exs
@@ -619,6 +619,9 @@ defmodule Ecto.Adapters.MyXQLTest do
 
     query = Schema |> select([r], r.x + 2) |> plan()
     assert all(query) == ~s{SELECT s0.`x` + 2 FROM `schema` AS s0}
+
+    query = Schema |> select([r], (not r.x) < r.y) |> plan()
+    assert all(query) == ~s{SELECT (NOT (s0.`x`)) < s0.`y` FROM `schema` AS s0}
   end
 
   test "is_nil" do
diff --git a/test/ecto/adapters/postgres_test.exs b/test/ecto/adapters/postgres_test.exs
index e877e7be..27d8e2c5 100644
--- a/test/ecto/adapters/postgres_test.exs
+++ b/test/ecto/adapters/postgres_test.exs
@@ -812,6 +812,9 @@ defmodule Ecto.Adapters.PostgresTest do
 
     query = Schema |> select([r], r.x + 2) |> plan()
     assert all(query) == ~s{SELECT s0."x" + 2 FROM "schema" AS s0}
+
+    query = Schema |> select([r], (not r.x) < r.y) |> plan()
+    assert all(query) == ~s{SELECT (NOT (s0."x")) < s0."y" FROM "schema" AS s0}
   end
 
   test "is_nil" do
diff --git a/test/ecto/adapters/tds_test.exs b/test/ecto/adapters/tds_test.exs
index 93413141..1f3f3b04 100644
--- a/test/ecto/adapters/tds_test.exs
+++ b/test/ecto/adapters/tds_test.exs
@@ -674,6 +674,9 @@ defmodule Ecto.Adapters.TdsTest do
 
     query = Schema |> select([r], r.x > 2) |> plan()
     assert all(query) == ~s{SELECT s0.[x] > 2 FROM [schema] AS s0}
+
+    query = Schema |> select([r], (not r.x) < r.y) |> plan()
+    assert all(query) == ~s{SELECT (NOT (s0.[x])) < s0.[y] FROM [schema] AS s0}
   end
 
   test "is_nil" do