The seventh-ranked Errani was a heavy favorite as she chased her seventh career title and first this year, but the 45th-ranked Barthel stayed on a roll after beating fifth-seeded Roberta Vinci and No. 3-seeded Marion Bartoli on her way to the final.
Barthel played aggressively, with 53 winners and 50 unforced errors. She clinched the victory with a forehand winner.
Her previous title came in Hobart, Australia, in January 2012, when she came through the qualifiers.