diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index d42fbfe1be..be2b86a531 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -289,15 +289,18 @@ def _resolve_build_config(defines: dict[str, str]) -> _BuildConfig:
                 "PIO_FRAMEWORK_ARDUINO_MMU_CUSTOM requires MMU_IRAM_SIZE and "
                 "MMU_ICACHE_SIZE build flags"
             )
-        for size_name in ("MMU_IRAM_SIZE", "MMU_ICACHE_SIZE"):
-            # These reach the linker-script preprocessor; a bare or
-            # non-numeric value would corrupt the segment lengths and fail
-            # far away in ld ("48K" and a valueless flag both preprocess
-            # wrong; ul suffixes survive preprocessing, see build_surgery)
-            value = defines[size_name].partition("=")[2]
-            if not re.fullmatch(r"(?:0[xX][0-9a-fA-F]+|\d+)[uUlL]*", value):
+        for name, body in defines.items():
+            if not name.startswith("MMU_") or "=" not in body:
+                # Valueless flags (MMU_IRAM_HEAP) are legitimate switches
+                continue
+            # Every valued MMU_* reaches the linker-script preprocessor; a
+            # bare or non-numeric value would corrupt the segment lengths
+            # and fail far away in ld. Hex only: build_surgery's segment
+            # parser (and upstream's spellings) cannot read decimal.
+            value = body.partition("=")[2]
+            if not re.fullmatch(r"0[xX][0-9a-fA-F]+[uUlL]*", value):
                 raise EsphomeError(
-                    f"{size_name} must be a numeric literal, got "
+                    f"{name} must be a hex literal (e.g. 0x8000), got "
                     f"{value or '(no value)'}"
                 )
         # Sorted so build.ninja and the linker-script stamp stay
@@ -305,11 +308,12 @@ def _resolve_build_config(defines: dict[str, str]) -> _BuildConfig:
         # iteration order).
         mmu = sorted(body for name, body in defines.items() if name.startswith("MMU_"))
     else:
-        if "MMU_IRAM_SIZE" in defines or "MMU_ICACHE_SIZE" in defines:
+        if raw := sorted(n for n in defines if n.startswith("MMU_")):
             # Unlike PlatformIO (whose defaults win the compile line), user
-            # MMU_* here would win the compile but not the linker script; refuse.
+            # MMU_* here would win the compile but not the linker script;
+            # refuse them all, like the knob branch above.
             raise EsphomeError(
-                "Custom MMU_IRAM_SIZE/MMU_ICACHE_SIZE build flags require "
+                f"Raw {', '.join(raw)} build flags require "
                 "-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM"
             )
         mmu = list(_MMU_DEFAULT)
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index efc7df4ded..4dd100f94a 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -794,7 +794,7 @@ def test_generate_ld_scripts_unreadable_note_still_warns(
     assert "could not be read" in caplog.text
 
 
-@pytest.mark.parametrize("value", ["0x8000", "0xC000ul", "32768", "48UL"])
+@pytest.mark.parametrize("value", ["0x8000", "0xC000ul", "0x10UL"])
 def test_mmu_custom_numeric_sizes_accepted(value: str) -> None:
     config = _resolve(
         "-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM",
@@ -804,11 +804,19 @@ def test_mmu_custom_numeric_sizes_accepted(value: str) -> None:
     assert f"MMU_IRAM_SIZE={value}" in config.mmu_defines
 
 
-@pytest.mark.parametrize("flag", ["-DMMU_IRAM_SIZE=48K", "-DMMU_IRAM_SIZE"])
+@pytest.mark.parametrize(
+    "flag",
+    [
+        "-DMMU_IRAM_SIZE=48K",
+        # Decimal passes preprocessing but build_surgery's segment parser
+        # only reads hex, so testing-mode surgery would fail misleadingly
+        "-DMMU_IRAM_SIZE=32768",
+    ],
+)
 def test_mmu_custom_malformed_size_raises(flag: str) -> None:
-    """A bare or non-numeric size would corrupt the preprocessed segment
-    lengths and fail far away in ld; refuse by name."""
-    with pytest.raises(EsphomeError, match="MMU_IRAM_SIZE must be a numeric"):
+    """A non-hex size would corrupt the preprocessed segment lengths (or
+    defeat the testing-mode surgery); refuse by name."""
+    with pytest.raises(EsphomeError, match="MMU_IRAM_SIZE must be a hex"):
         _resolve(
             "-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM",
             flag,
@@ -816,6 +824,32 @@ def test_mmu_custom_malformed_size_raises(flag: str) -> None:
         )
 
 
+def test_mmu_custom_valueless_switch_accepted_and_others_validated() -> None:
+    """Valueless MMU switches (MMU_IRAM_HEAP) pass; every valued MMU_* is
+    hex-validated, not just the two required sizes."""
+    config = _resolve(
+        "-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM",
+        "-DMMU_IRAM_SIZE=0x8000",
+        "-DMMU_ICACHE_SIZE=0x8000",
+        "-DMMU_IRAM_HEAP",
+    )
+    assert "MMU_IRAM_HEAP" in config.mmu_defines
+    with pytest.raises(EsphomeError, match="MMU_SEC_HEAP_SIZE must be a hex"):
+        _resolve(
+            "-DPIO_FRAMEWORK_ARDUINO_MMU_CUSTOM",
+            "-DMMU_IRAM_SIZE=0x8000",
+            "-DMMU_ICACHE_SIZE=0x8000",
+            "-DMMU_SEC_HEAP_SIZE=48K",
+        )
+
+
+def test_mmu_no_knob_rejects_any_raw_mmu_flag() -> None:
+    """The no-knob branch refuses every raw MMU_*, like the knob branch; a
+    lone switch would win the compile line but not the linker script."""
+    with pytest.raises(EsphomeError, match="Raw MMU_IRAM_HEAP"):
+        _resolve("-DMMU_IRAM_HEAP")
+
+
 def test_raw_nonosdk_define_raises() -> None:
     """A raw NONOSDK* define would split the compile line from the linked
     SDK libraries, like the lwIP knob overrides."""