Barthel, 23, made the most of an open draw which had seen Serena Williams withdraw and seven of the top eight seeds losing in the first round.
It meant the Båstad event was the first WTA tournament since Hobart in 2009 where no seeds had made the quarter-finals.
"It's been an amazing week for me — I cannot believe I won the title," said world number 61 Barthel whose two previous titles came in Hobart in 2012 and Paris in 2013.
World number one Serena Williams was forced to pull out before the tournament began after failing to recover from the illness which dramatically ended her Wimbledon campaign.
The 32-year-old American star, who was the defending champion at the Båstad claycourt event, withdrew from the Wimbledon doubles event after suffering from what the WTA described as a "viral illness".
"Båstad was my favourite tournament I played in last year, thus I am very disappointed not to come back there. But there is still next year, so I can't wait to go back there."