Barthel broke Pliskova for a 4-2 lead in the final set before converting her second match point for her fourth WTA title and the first since 2014.
"I can't believe it," Barthel said. "Somehow, I made it. I'm completely exhausted."
The 82nd-ranked Barthel saved three match points in the second round of qualifying on her way to victory in the final.
Barthel dropped her serve Saturday to go 3-4 down in the second set but she got a break back in the following game. The German wasted a set point at 5-4 but converted her second set point to take the set 7-5.
Barthel improved her record against the 58th-ranked Czech to 2-1.
In the first set, Barthel dropped her serve in the opening game with two straight double faults. Pliskova broke her twice more before serving out the set.
"I'm sorry I didn't make it," said Pliskova who was seeking her second WTA title.