diff --git a/docs/changelog/2182.bugfix.rst b/docs/changelog/2182.bugfix.rst
new file mode 100644
index 000000000..a78577254
--- /dev/null
+++ b/docs/changelog/2182.bugfix.rst
@@ -0,0 +1 @@
+Fix env variable substitutions with defaults containing colon (e.g. URL) -- by :user:`comabrewer`.
diff --git a/src/tox/config/loader/ini/replace.py b/src/tox/config/loader/ini/replace.py
index ca8c4a32c..b2735c582 100644
--- a/src/tox/config/loader/ini/replace.py
+++ b/src/tox/config/loader/ini/replace.py
@@ -207,7 +207,7 @@ def replace_env(conf: "Config", env_name: Optional[str], args: List[str], chain:
     if key in os.environ:
         return os.environ[key]
 
-    return "" if len(args) == 1 else args[1]
+    return "" if len(args) == 1 else ":".join(args[1:])
 
 
 def replace_tty(args: List[str]) -> str:
diff --git a/tests/config/loader/ini/replace/test_replace_env_var.py b/tests/config/loader/ini/replace/test_replace_env_var.py
index 3a298d79c..1ce8fc1c4 100644
--- a/tests/config/loader/ini/replace/test_replace_env_var.py
+++ b/tests/config/loader/ini/replace/test_replace_env_var.py
@@ -36,3 +36,10 @@ def test_replace_env_var_circular(replace_one: ReplaceOne, monkeypatch: MonkeyPa
     monkeypatch.setenv("MAGIC", "{env:MAGIC}")
     result = replace_one("{env:MAGIC}")
     assert result == "{env:MAGIC}"
+
+
+def test_replace_env_default_with_colon(replace_one: ReplaceOne, monkeypatch: MonkeyPatch) -> None:
+    """If we have a factor that is not specified within the core env-list then that's also an environment"""
+    monkeypatch.delenv("MAGIC", raising=False)
+    result = replace_one("{env:MAGIC:https://some.url.org}")
+    assert result == "https://some.url.org"