It was the second WTA title for Barthel in three finals appearances. She won the Hobart, Australia, WTA stop in 2012 and was runner-up in that tournament last month. Errani is now 6-5 in championship matches.
Barthel cashed in both of her break chances in taking the first set and after both players had two breaks in second, came away with the win in the tiebreaker.
Errani was good on 83 percent of her first serves but Barthel still won nearly half of the points off Errani's serve.
Barthel picked up the champion's check for about $129,000, pushing her career winnings to more than $750,000. Errani, who is seventh in the world rankings, won $68,500 as runner-up.