The German was a surprise finalist 12 months ago but lost out to countrywoman Angelique Kerber.
A repeat looked on the cards when France's Kristina Mladenovic led 4-1 in the deciding tie-break but Siegemund fought back to win 6-1 2-6 7-6 (7/5).
Mladenovic ended Maria Sharapova's comeback tournament in the semi-finals but the Frenchwoman lost the opening set.
Mladenovic responded well to claim the second set but Siegemund, a wild card ranked 49, regained control in the decider and gave herself the chance to serve for the title at 5-4.
There was controversy as umpire Mariana Alves gave a point penalty against Siegemund for a time violation and Mladenovic broke back.
But ultimately there was no denying the 29-year-old, who was able to celebrate winning her first Premier level tournament.
Elina Svitolina won her third title of the season at the Istanbul Cup.
The top-seeded Ukrainian dropped only one set all week and saw off Elise Mertens of Belgium in the final 6-2 6-4.
Read the original article on Eurosport: Laura Siegemund claims Porsche Grand Prix title in Stuttgart