diff --git a/docs/changelog/2822.bugfix.rst b/docs/changelog/2822.bugfix.rst
new file mode 100644
index 000000000..3c0dc9d8e
--- /dev/null
+++ b/docs/changelog/2822.bugfix.rst
@@ -0,0 +1 @@
+Require space after colon before factor filter expansion, unless it is the last character of the line - by :user:`pdecat`.
diff --git a/src/tox/config/loader/ini/factor.py b/src/tox/config/loader/ini/factor.py
index e13b5ec95..c429efb1d 100644
--- a/src/tox/config/loader/ini/factor.py
+++ b/src/tox/config/loader/ini/factor.py
@@ -50,7 +50,7 @@ def expand_factors(value: str) -> Iterator[tuple[list[list[tuple[str, bool]]] |
     for line in value.split("\n"):
         factors: list[list[tuple[str, bool]]] | None = None
         marker_at, content = line.find(":"), line
-        if marker_at != -1:
+        if marker_at != -1 and (len(line) == marker_at + 1 or line[marker_at + 1] == " "):
             try:
                 factors = list(find_factor_groups(line[:marker_at].strip()))
             except ValueError:
diff --git a/tests/config/loader/ini/test_factor.py b/tests/config/loader/ini/test_factor.py
index 00aa83c94..cec786b3f 100644
--- a/tests/config/loader/ini/test_factor.py
+++ b/tests/config/loader/ini/test_factor.py
@@ -30,6 +30,7 @@ def complex_example() -> str:
     py, d: space
     extra: extra
     more-default
+    no:space
     """,
     )
 
@@ -73,6 +74,7 @@ def test_factor_env_filter(env: str, complex_example: str) -> None:
     assert "default" in result
     assert "lines" in result
     assert "more-default" in result
+    assert "no:space" in result
     if "py" in env:
         assert "py only" in result
         assert "not py" not in result