diff --git a/tests/integration/fixtures/external_components/legacy_climate_component/__init__.py b/tests/integration/fixtures/external_components/legacy_climate_component/__init__.py
new file mode 100644
index 0000000000..a495fb81ad
--- /dev/null
+++ b/tests/integration/fixtures/external_components/legacy_climate_component/__init__.py
@@ -0,0 +1 @@
+"""Legacy climate component — tests deprecated ClimateTraits setters backward compat."""
diff --git a/tests/integration/fixtures/external_components/legacy_climate_component/climate/__init__.py b/tests/integration/fixtures/external_components/legacy_climate_component/climate/__init__.py
new file mode 100644
index 0000000000..08ef4f542a
--- /dev/null
+++ b/tests/integration/fixtures/external_components/legacy_climate_component/climate/__init__.py
@@ -0,0 +1,15 @@
+"""Legacy climate platform that uses deprecated ClimateTraits setters."""
+
+import esphome.codegen as cg
+from esphome.components import climate
+import esphome.config_validation as cv
+
+legacy_climate_ns = cg.esphome_ns.namespace("legacy_climate_test")
+LegacyClimate = legacy_climate_ns.class_("LegacyClimate", climate.Climate, cg.Component)
+
+CONFIG_SCHEMA = climate.climate_schema(LegacyClimate).extend(cv.COMPONENT_SCHEMA)
+
+
+async def to_code(config):
+    var = await climate.new_climate(config)
+    await cg.register_component(var, config)
diff --git a/tests/integration/fixtures/external_components/legacy_climate_component/climate/legacy_climate.h b/tests/integration/fixtures/external_components/legacy_climate_component/climate/legacy_climate.h
new file mode 100644
index 0000000000..b2fea53dd6
--- /dev/null
+++ b/tests/integration/fixtures/external_components/legacy_climate_component/climate/legacy_climate.h
@@ -0,0 +1,59 @@
+#pragma once
+
+#include "esphome/components/climate/climate.h"
+#include "esphome/core/component.h"
+
+namespace esphome {
+namespace legacy_climate_test {
+
+/// Test climate that uses the DEPRECATED ClimateTraits setters for custom modes.
+/// This validates backward compatibility for external components that haven't migrated.
+class LegacyClimate : public climate::Climate, public Component {
+ public:
+  void setup() override {
+    this->mode = climate::CLIMATE_MODE_OFF;
+    this->target_temperature = 22.0f;
+    this->current_temperature = 20.0f;
+    this->publish_state();
+  }
+
+  float get_setup_priority() const override { return setup_priority::LATE; }
+
+ protected:
+  climate::ClimateTraits traits() override {
+    auto traits = climate::ClimateTraits();
+    traits.add_feature_flags(climate::CLIMATE_SUPPORTS_CURRENT_TEMPERATURE);
+    traits.set_supported_modes({climate::CLIMATE_MODE_OFF, climate::CLIMATE_MODE_HEAT, climate::CLIMATE_MODE_COOL});
+    traits.set_visual_min_temperature(16.0f);
+    traits.set_visual_max_temperature(30.0f);
+    traits.set_visual_temperature_step(0.5f);
+
+    // DEPRECATED API: setting custom modes directly on ClimateTraits.
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+    traits.set_supported_custom_fan_modes({"Turbo", "Silent", "Auto"});
+    traits.set_supported_custom_presets({"Eco Mode", "Night Mode"});
+#pragma GCC diagnostic pop
+
+    return traits;
+  }
+
+  void control(const climate::ClimateCall &call) override {
+    if (call.get_mode().has_value()) {
+      this->mode = *call.get_mode();
+    }
+    if (call.get_target_temperature().has_value()) {
+      this->target_temperature = *call.get_target_temperature();
+    }
+    if (call.get_custom_fan_mode() != nullptr) {
+      this->set_custom_fan_mode_(call.get_custom_fan_mode());
+    }
+    if (call.get_custom_preset() != nullptr) {
+      this->set_custom_preset_(call.get_custom_preset());
+    }
+    this->publish_state();
+  }
+};
+
+}  // namespace legacy_climate_test
+}  // namespace esphome
diff --git a/tests/integration/fixtures/legacy_climate_compat.yaml b/tests/integration/fixtures/legacy_climate_compat.yaml
new file mode 100644
index 0000000000..13f88aab02
--- /dev/null
+++ b/tests/integration/fixtures/legacy_climate_compat.yaml
@@ -0,0 +1,21 @@
+esphome:
+  name: legacy-climate-compat
+  platformio_options:
+    build_flags:
+      - "-DUSE_HOST"
+
+host:
+api:
+logger:
+  level: DEBUG
+
+external_components:
+  - source:
+      type: local
+      path: EXTERNAL_COMPONENT_PATH
+    components: [legacy_climate_component]
+
+climate:
+  - platform: legacy_climate_component
+    name: "Legacy Climate"
+    id: legacy_climate
diff --git a/tests/integration/test_legacy_climate_compat.py b/tests/integration/test_legacy_climate_compat.py
new file mode 100644
index 0000000000..430778134d
--- /dev/null
+++ b/tests/integration/test_legacy_climate_compat.py
@@ -0,0 +1,93 @@
+"""Integration test for backward compatibility of deprecated ClimateTraits setters.
+
+Verifies that external components using the old traits.set_supported_custom_fan_modes()
+and traits.set_supported_custom_presets() API still work correctly during the
+deprecation period (removed in 2026.11.0).
+"""
+
+from __future__ import annotations
+
+import asyncio
+from pathlib import Path
+
+import aioesphomeapi
+from aioesphomeapi import ClimateInfo
+import pytest
+
+from .state_utils import InitialStateHelper
+from .types import APIClientConnectedFactory, RunCompiledFunction
+
+
+@pytest.mark.asyncio
+async def test_legacy_climate_compat(
+    yaml_config: str,
+    run_compiled: RunCompiledFunction,
+    api_client_connected: APIClientConnectedFactory,
+) -> None:
+    """Test that deprecated ClimateTraits custom mode setters still work end-to-end."""
+    external_components_path = str(
+        Path(__file__).parent / "fixtures" / "external_components"
+    )
+    yaml_config = yaml_config.replace(
+        "EXTERNAL_COMPONENT_PATH", external_components_path
+    )
+
+    loop = asyncio.get_running_loop()
+
+    async with run_compiled(yaml_config), api_client_connected() as client:
+        entities, services = await client.list_entities_services()
+        initial_state_helper = InitialStateHelper(entities)
+
+        climate_infos = [e for e in entities if isinstance(e, ClimateInfo)]
+        assert len(climate_infos) == 1, (
+            f"Expected 1 climate entity, got {len(climate_infos)}"
+        )
+
+        test_climate = climate_infos[0]
+
+        # Verify custom fan modes set via deprecated ClimateTraits setter are exposed
+        assert set(test_climate.supported_custom_fan_modes) == {
+            "Turbo",
+            "Silent",
+            "Auto",
+        }, (
+            f"Expected custom fan modes {{Turbo, Silent, Auto}}, "
+            f"got {test_climate.supported_custom_fan_modes}"
+        )
+
+        # Verify custom presets set via deprecated ClimateTraits setter are exposed
+        assert set(test_climate.supported_custom_presets) == {
+            "Eco Mode",
+            "Night Mode",
+        }, (
+            f"Expected custom presets {{Eco Mode, Night Mode}}, "
+            f"got {test_climate.supported_custom_presets}"
+        )
+
+        # Set up state tracking with InitialStateHelper
+        turbo_future: asyncio.Future[aioesphomeapi.ClimateState] = loop.create_future()
+
+        def on_state(state: aioesphomeapi.EntityState) -> None:
+            if (
+                isinstance(state, aioesphomeapi.ClimateState)
+                and state.custom_fan_mode == "Turbo"
+                and not turbo_future.done()
+            ):
+                turbo_future.set_result(state)
+
+        client.subscribe_states(initial_state_helper.on_state_wrapper(on_state))
+
+        try:
+            await initial_state_helper.wait_for_initial_states()
+        except TimeoutError:
+            pytest.fail("Timeout waiting for initial states")
+
+        # Verify we can set a custom fan mode via API (tests find_custom_fan_mode_ compat path)
+        client.climate_command(test_climate.key, custom_fan_mode="Turbo")
+
+        try:
+            turbo_state = await asyncio.wait_for(turbo_future, timeout=5.0)
+        except TimeoutError:
+            pytest.fail("Custom fan mode 'Turbo' not received within 5 seconds")
+
+        assert turbo_state.custom_fan_mode == "Turbo"