Flo (Swedish: Flo socken) is a historic parish (socken) in the Swedish province of Västergötland.

Since 1971 it has been a part of Grästorp Municipality.

Flo parish is the namesake for the Floian stage, which is the second stage of the Lower Ordovician.

[1] The base of the Floian is defined at the Diabasbrottet quarry type section (GSSP) 5 km to the northwest of Flo.

The base is defined in the quarry as the lowest stratigraphic occurrence of the graptolite species Tetragraptus approximatus.