diff --git a/esphome/core/helpers.cpp b/esphome/core/helpers.cpp
index ded8051df8..71e3c87e1e 100644
--- a/esphome/core/helpers.cpp
+++ b/esphome/core/helpers.cpp
@@ -233,6 +233,26 @@ bool str_contains_ignore_case_fallback(const char *haystack, const char *needle)
   return false;
 }
 
+#ifdef USE_ESP8266
+// _P mirror of str_contains_ignore_case_fallback above; host tests cover only the fallback,
+// so keep the two bodies in sync.
+bool str_contains_ignore_case_p(const char *haystack, PGM_P needle) {
+  if (haystack == nullptr || needle == nullptr) {
+    return false;
+  }
+  const size_t needle_len = strlen_P(needle);
+  if (needle_len == 0) {
+    return true;
+  }
+  for (const char *p = haystack; *p != '\0'; p++) {
+    if (strncasecmp_P(p, needle, needle_len) == 0) {
+      return true;
+    }
+  }
+  return false;
+}
+#endif  // USE_ESP8266
+
 // str_truncate, str_until, str_lower_case, str_upper_case, str_snake_case moved to alloc_helpers.cpp
 char *str_sanitize_to(char *buffer, size_t buffer_size, const char *str) {
   if (buffer_size == 0) {
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index b13d92ccce..e60316d4ee 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -984,6 +984,15 @@ inline bool str_endswith_ignore_case(const std::string &str, const char *suffix)
 /// Fallback implementation for case insensitive substring comparison.
 bool str_contains_ignore_case_fallback(const char *haystack, const char *needle);
 
+#ifdef USE_ESP8266
+/// ESP8266 internal implementation reading the needle from flash — prefer the
+/// `str_contains_ignore_case` macro which wraps needle literals with PSTR() automatically.
+bool str_contains_ignore_case_p(const char *haystack, PGM_P needle);
+/// Case-insensitive check if needle string is contained in haystack (no heap allocation).
+/// On ESP8266 the needle is wrapped with PSTR() so it stays in flash, which requires it to be
+/// a string literal; a runtime needle needs str_contains_ignore_case_p behind #ifdef USE_ESP8266.
+#define str_contains_ignore_case(haystack, needle) str_contains_ignore_case_p(haystack, PSTR(needle))
+#else
 /// Case-insensitive check if needle string is contained in haystack (no heap allocation).
 inline bool str_contains_ignore_case(const char *haystack, const char *needle) {
   if (!needle || !haystack) {
@@ -991,7 +1000,7 @@ inline bool str_contains_ignore_case(const char *haystack, const char *needle) {
   }
 
 // strcasestr is a GNU extension: newlib only declares it when _GNU_SOURCE is set.
-// ESP32/ESP8266/host builds get it from their framework or from g++ on Linux;
+// ESP32/host builds get it from their framework or from g++ on Linux;
 // LibreTiny, RP2 and Zephyr do not, so they use the hand-rolled fallback.
 #if defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
   return str_contains_ignore_case_fallback(haystack, needle);
@@ -999,6 +1008,7 @@ inline bool str_contains_ignore_case(const char *haystack, const char *needle) {
   return strcasestr(haystack, needle) != nullptr;
 #endif  // defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
 }
+#endif  // USE_ESP8266
 
 // str_truncate moved to alloc_helpers.h - remove this include before 2026.11.0
 
diff --git a/tests/components/core/helpers_test.cpp b/tests/components/core/helpers_test.cpp
index d5219f9d47..521ae10e5c 100644
--- a/tests/components/core/helpers_test.cpp
+++ b/tests/components/core/helpers_test.cpp
@@ -121,7 +121,7 @@ TEST(StringContainsIgnoreCaseTest, MiscNotMatching) {
 
 TEST(StringContainsIgnoreCaseTest, FallbackMatchesLibc) {
   const char *haystack = "Hello World";
-  for (const char *needle : {"", "Hello", "hELLO", "Hell", "world", "Heaven", "Hello!", "d"}) {
+  for (const char *needle : {"", "Hello", "hELLO", "HELLO", "Hell", "world", "World", "Heaven", "Hello!", "d"}) {
     EXPECT_EQ(str_contains_ignore_case_fallback(haystack, needle), str_contains_ignore_case(haystack, needle))
         << "needle: " << needle;
   }