diff --git a/esphome/core/log.cpp b/esphome/core/log.cpp
index c103bf331e..f92b59fe13 100644
--- a/esphome/core/log.cpp
+++ b/esphome/core/log.cpp
@@ -17,6 +17,7 @@ namespace esphome {
 // tests/component_tests/logger/test_logger.py.
 void HOT esp_log_printf_(int level, const char *tag, int line, const char *format, ...) {  // NOLINT
 #ifdef USE_LOGGER
+  ESPHOME_DEBUG_ASSERT(logger::global_logger != nullptr);
   va_list arg;
   va_start(arg, format);
   logger::global_logger->log_vprintf_(static_cast<uint8_t>(level), tag, line, format, arg);
@@ -27,6 +28,7 @@ void HOT esp_log_printf_(int level, const char *tag, int line, const char *forma
 #ifdef USE_STORE_LOG_STR_IN_FLASH
 void HOT esp_log_printf_(int level, const char *tag, int line, const __FlashStringHelper *format, ...) {
 #ifdef USE_LOGGER
+  ESPHOME_DEBUG_ASSERT(logger::global_logger != nullptr);
   va_list arg;
   va_start(arg, format);
   logger::global_logger->log_vprintf_(static_cast<uint8_t>(level), tag, line, format, arg);
@@ -37,6 +39,7 @@ void HOT esp_log_printf_(int level, const char *tag, int line, const __FlashStri
 
 void HOT esp_log_vprintf_(int level, const char *tag, int line, const char *format, va_list args) {  // NOLINT
 #ifdef USE_LOGGER
+  ESPHOME_DEBUG_ASSERT(logger::global_logger != nullptr);
   logger::global_logger->log_vprintf_(static_cast<uint8_t>(level), tag, line, format, args);
 #endif
 }
@@ -44,6 +47,7 @@ void HOT esp_log_vprintf_(int level, const char *tag, int line, const char *form
 #ifdef USE_ESP32
 int HOT esp_idf_log_vprintf_(const char *format, va_list args) {  // NOLINT
 #ifdef USE_LOGGER
+  ESPHOME_DEBUG_ASSERT(logger::global_logger != nullptr);
   logger::global_logger->log_vprintf_(ESPHOME_LOG_LEVEL, "esp-idf", 0, format, args);
 #endif
   return 0;
diff --git a/esphome/core/log.h b/esphome/core/log.h
index 14a0cb0572..134e816150 100644
--- a/esphome/core/log.h
+++ b/esphome/core/log.h
@@ -4,6 +4,14 @@
 
 #include <cassert>
 #include <cstdarg>
+
+// Debug assert that only fires when ESPHOME_DEBUG is defined (e.g. in CI/test builds).
+// Zero cost in production firmware.
+#ifdef ESPHOME_DEBUG
+#define ESPHOME_DEBUG_ASSERT(expr) assert(expr)  // NOLINT
+#else
+#define ESPHOME_DEBUG_ASSERT(expr) ((void) 0)
+#endif
 // for PRIu32 and friends
 #include <cinttypes>
 #include <string>
diff --git a/script/cpp_unit_test.py b/script/cpp_unit_test.py
index b87261ab33..6ba4127848 100755
--- a/script/cpp_unit_test.py
+++ b/script/cpp_unit_test.py
@@ -100,6 +100,7 @@ def create_test_config(config_name: str, includes: list[str]) -> dict:
                 "build_flags": [
                     "-Og",  # optimize for debug
                     "-DUSE_TIME_TIMEZONE",  # enable timezone code paths for testing
+                    "-DESPHOME_DEBUG",  # enable debug assertions
                 ],
                 "debug_build_flags": [  # only for debug builds
                     "-g3",  # max debug info