diff --git a/esphome/build_gen/arduino8266.py b/esphome/build_gen/arduino8266.py
index e29ffca66d..bccdb23d0d 100644
--- a/esphome/build_gen/arduino8266.py
+++ b/esphome/build_gen/arduino8266.py
@@ -176,10 +176,15 @@ def _flag_defines() -> dict[str, str]:
     defines: dict[str, str] = {}
     for flag in CORE.build_flags:
         # Shell-lex multi-token entries the way PlatformIO does, so a knob
-        # in "-DKNOB -DOTHER" is still detected; single tokens pass verbatim
-        # to keep any quoting in their bodies intact.
-        for tok in split_flag_entry(flag, "esphome") if " " in flag else (flag,):
-            if tok.startswith("-D"):
+        # in "-DKNOB -DOTHER" or a spaced "-D KNOB" is still detected;
+        # single tokens pass verbatim to keep quoting in their bodies intact.
+        tokens = (
+            join_flag_args(split_flag_entry(flag, "esphome"), "esphome")
+            if " " in flag
+            else (flag,)
+        )
+        for tok in tokens:
+            if tok.startswith("-D") and len(tok) > 2:
                 body = tok[2:]
                 defines[body.split("=", 1)[0]] = body
     return defines
diff --git a/esphome/platformio/library.py b/esphome/platformio/library.py
index c1e659e0f6..753bbdd8a3 100644
--- a/esphome/platformio/library.py
+++ b/esphome/platformio/library.py
@@ -564,11 +564,12 @@ def split_flag_entry(entry: str, owner: str) -> list[str]:
 
 
 def join_flag_args(tokens: Iterable[str], owner: str) -> list[str]:
-    """Join a bare ``-I``/``-L``/``-l`` with its following token (PIO lexing)."""
+    """Join a bare ``-I``/``-L``/``-l``/``-D`` with its following token,
+    the way PlatformIO's ParseFlags lexes them."""
     out: list[str] = []
     it = iter(tokens)
     for tok in it:
-        if tok in ("-I", "-L", "-l"):
+        if tok in ("-I", "-L", "-l", "-D"):
             arg = next(it, None)
             if arg is None:
                 _LOGGER.warning("Ignoring trailing '%s' in %s build flags", tok, owner)
diff --git a/tests/unit_tests/build_gen/test_arduino8266.py b/tests/unit_tests/build_gen/test_arduino8266.py
index f5d1368c94..a511226867 100644
--- a/tests/unit_tests/build_gen/test_arduino8266.py
+++ b/tests/unit_tests/build_gen/test_arduino8266.py
@@ -665,3 +665,11 @@ def test_write_project_empty_core_raises(tmp_path: Path) -> None:
         pytest.raises(EsphomeError, match="no core sources"),
     ):
         arduino8266.write_project(paths)
+
+
+def test_flag_defines_joins_spaced_define() -> None:
+    """A spaced "-D KNOB" entry is detected exactly as PlatformIO detects it."""
+    _set_flags("-D PIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH")
+    defines = _flag_defines()
+    assert "PIO_FRAMEWORK_ARDUINO_LWIP2_HIGHER_BANDWIDTH_LOW_FLASH" in defines
+    assert "" not in defines
diff --git a/tests/unit_tests/test_platformio_library.py b/tests/unit_tests/test_platformio_library.py
index 6c7f6d6462..25af9f6794 100644
--- a/tests/unit_tests/test_platformio_library.py
+++ b/tests/unit_tests/test_platformio_library.py
@@ -538,5 +538,9 @@ def test_split_flag_entry_unbalanced_quote_is_clean() -> None:
     from esphome.platformio.library import split_flag_entry
 
     assert split_flag_entry('-DX="a b"', "library x") == ["-DX=a b"]
+    # join_flag_args re-glues a spaced -D like ParseFlags does
+    from esphome.platformio.library import join_flag_args
+
+    assert join_flag_args(["-D", "FOO=1", "-Os"], "x") == ["-DFOO=1", "-Os"]
     with pytest.raises(EsphomeError, match=r"Malformed build flag.*library x"):
         split_flag_entry('-DX="unclosed', "library x")