diff --git a/colorlog/formatter.py b/colorlog/formatter.py
index e4fb14d..f67d44e 100644
--- a/colorlog/formatter.py
+++ b/colorlog/formatter.py
@@ -212,6 +212,12 @@ def __init__(self, fmt: typing.Mapping[str, str], **kwargs: typing.Any) -> None:
         - fmt (dict):
             A mapping of log levels (represented as strings, e.g. 'WARNING') to
             format strings. (*New in version 2.7.0)
+
+            Levels that are not present in the mapping fall back to a default
+            formatter, so records logged at custom or unlisted levels are
+            formatted rather than raising a ``KeyError``. To customise the
+            fallback, provide a format string under the special ``"DEFAULT"``
+            key; otherwise the default format for the chosen ``style`` is used.
         (All other parameters are the same as in colorlog.ColoredFormatter)
 
         Example:
@@ -229,9 +235,15 @@ def __init__(self, fmt: typing.Mapping[str, str], **kwargs: typing.Any) -> None:
         self.formatters = {
             level: ColoredFormatter(fmt=f, **kwargs) for level, f in fmt.items()
         }
+        # Used for any level not present in ``fmt``. An explicit "DEFAULT" entry
+        # takes precedence; otherwise fall back to the default format string.
+        self.default_formatter = self.formatters.get(
+            "DEFAULT", ColoredFormatter(**kwargs)
+        )
 
     def format(self, record: logging.LogRecord) -> str:
-        return self.formatters[record.levelname].format(record)
+        formatter = self.formatters.get(record.levelname, self.default_formatter)
+        return formatter.format(record)
 
 
 # Provided for backwards compatibility. The features provided by this subclass are now
diff --git a/colorlog/tests/test_colorlog.py b/colorlog/tests/test_colorlog.py
index 23270ce..8ac11e1 100644
--- a/colorlog/tests/test_colorlog.py
+++ b/colorlog/tests/test_colorlog.py
@@ -1,5 +1,6 @@
 """Test the colorlog.colorlog module."""
 
+import logging
 import sys
 import unittest.mock
 
@@ -100,6 +101,29 @@ def test_level_formatter(self, create_and_test_logger):
             },
         )
 
+    def test_unlisted_level_uses_default(self):
+        """A level missing from fmt must not raise; it uses the fallback."""
+        logging.addLevelName(25, "NOTICE")
+        formatter = colorlog.LevelFormatter(fmt={"INFO": "%(message)s"})
+        record = logging.LogRecord(
+            "test", 25, "path", 1, "a notice message", None, None
+        )
+        assert "a notice message" in formatter.format(record)
+
+    def test_explicit_default_key(self):
+        """A 'DEFAULT' entry overrides the fallback format for unlisted levels."""
+        logging.addLevelName(25, "NOTICE")
+        formatter = colorlog.LevelFormatter(
+            fmt={
+                "INFO": "%(message)s",
+                "DEFAULT": "DEFAULT: %(message)s",
+            }
+        )
+        record = logging.LogRecord(
+            "test", 25, "path", 1, "a notice message", None, None
+        )
+        assert "DEFAULT: a notice message" in formatter.format(record)
+
 
 def test_ttycolorlog(create_and_test_logger, monkeypatch):
     monkeypatch.setattr(sys.stderr, "isatty", lambda: True)