diff --git a/docs/changelog/2823.bugfix.rst b/docs/changelog/2823.bugfix.rst
new file mode 100644
index 000000000..7b0b6e21c
--- /dev/null
+++ b/docs/changelog/2823.bugfix.rst
@@ -0,0 +1 @@
+Also accept tab after colon before factor filter expansion - by :user:`pdecat`.
diff --git a/src/tox/config/loader/ini/factor.py b/src/tox/config/loader/ini/factor.py
index c429efb1d..099948cda 100644
--- a/src/tox/config/loader/ini/factor.py
+++ b/src/tox/config/loader/ini/factor.py
@@ -49,8 +49,9 @@ def explode_factor(group: list[tuple[str, bool]]) -> str:
 def expand_factors(value: str) -> Iterator[tuple[list[list[tuple[str, bool]]] | None, str]]:
     for line in value.split("\n"):
         factors: list[list[tuple[str, bool]]] | None = None
-        marker_at, content = line.find(":"), line
-        if marker_at != -1 and (len(line) == marker_at + 1 or line[marker_at + 1] == " "):
+        marker_search = re.search(r":(\s|$)", line)
+        marker_at, content = marker_search.start() if marker_search else -1, line
+        if 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 cec786b3f..2a6f405c4 100644
--- a/tests/config/loader/ini/test_factor.py
+++ b/tests/config/loader/ini/test_factor.py
@@ -31,6 +31,8 @@ def complex_example() -> str:
     extra: extra
     more-default
     no:space
+    trailingcolon:
+    tab:\ttab
     """,
     )
 
@@ -50,6 +52,8 @@ def test_factor_env_discover(complex_example: str) -> None:
         "c",
         "d",
         "extra",
+        "trailingcolon",
+        "tab",
     ]
 
 
@@ -67,6 +71,8 @@ def test_factor_env_discover(complex_example: str) -> None:
         "pi-b-dev",
         "c",
         "extra",
+        "trailingcolon",
+        "tab",
     ],
 )
 def test_factor_env_filter(env: str, complex_example: str) -> None: