diff --git a/esphome/build_gen/espidf.py b/esphome/build_gen/espidf.py
index 5ad2072c5b3..96f84ebbd14 100644
--- a/esphome/build_gen/espidf.py
+++ b/esphome/build_gen/espidf.py
@@ -3,7 +3,8 @@
 import json
 from pathlib import Path
 
-from esphome.components.esp32 import get_esp32_variant
+from esphome.components.esp32 import get_esp32_variant, idf_version
+import esphome.config_validation as cv
 from esphome.core import CORE
 from esphome.helpers import mkdir_p, write_file_if_changed
 from esphome.writer import update_storage_json
@@ -61,6 +62,11 @@ def get_project_cmakelists(minimal: bool = False) -> str:
     variant = get_esp32_variant()
     idf_target = variant.lower().replace("-", "")
 
+    # esp_idf_size 2.x (bundled with IDF >=6.0) made NG the default and
+    # removed the --ng flag; on 1.x (IDF 5.5) --ng is required to get
+    # --format=raw because the legacy mode doesn't support it.
+    size_ng_flag = "--ng" if idf_version() < cv.Version(6, 0, 0) else ""
+
     # Project-wide compile options: -D defines and -W warning flags (skip
     # -Wl, linker flags — those go on the src component via
     # target_link_options below). Emitted via idf_build_set_property so the
@@ -146,7 +152,7 @@ project({CORE.name})
 # Emit raw JSON size data for ESPHome to read post-build.
 add_custom_command(
     TARGET ${{CMAKE_PROJECT_NAME}}.elf POST_BUILD
-    COMMAND ${{PYTHON}} -m esp_idf_size --ng --format=raw
+    COMMAND ${{PYTHON}} -m esp_idf_size {size_ng_flag} --format=raw
             -o ${{CMAKE_BINARY_DIR}}/esp_idf_size.json
             ${{CMAKE_PROJECT_NAME}}.map
     WORKING_DIRECTORY ${{CMAKE_BINARY_DIR}}
diff --git a/tests/unit_tests/build_gen/test_espidf.py b/tests/unit_tests/build_gen/test_espidf.py
index 36f04423557..540dd067318 100644
--- a/tests/unit_tests/build_gen/test_espidf.py
+++ b/tests/unit_tests/build_gen/test_espidf.py
@@ -11,10 +11,12 @@ import pytest
 from esphome.components.esp32 import (
     KEY_COMPONENTS,
     KEY_ESP32,
+    KEY_IDF_VERSION,
     KEY_PATH,
     KEY_REF,
     KEY_REPO,
 )
+import esphome.config_validation as cv
 from esphome.const import KEY_CORE
 from esphome.core import CORE
 
@@ -24,7 +26,10 @@ def _reset_core(tmp_path: Path) -> None:
     """Give each test its own CORE.build_path and a clean esp32 data slot."""
     CORE.build_path = str(tmp_path)
     CORE.data.setdefault(KEY_CORE, {})
-    CORE.data[KEY_ESP32] = {KEY_COMPONENTS: {}}
+    CORE.data[KEY_ESP32] = {
+        KEY_COMPONENTS: {},
+        KEY_IDF_VERSION: cv.Version(5, 5, 4),
+    }
 
 
 def _write_project_description(tmp_path: Path, components: dict[str, str]) -> None: