World number 87 Paolini had already eliminated three seeded players to reach her first final and she recovered from a slow start to see off another 7-6 (7-4) 6-2.
Third seed Riske led 5-2 in the first set after breaking her opponent's serve three times in the opening seven games, but Paolini dug deep and took four games on the spin.
The Italian used that momentum in the tie-break to open up a 5-2 advantage before getting over the line with her second set point.
After a gruelling 63-minute opening set, Paolini found the second a more straightforward affair as she broke for 3-2 and reeled off the remaining games, the last of those points coming via an error-forcing forehand.
Paolini, who won two-thirds of her second-service points and converted six of her 10 break points, is glad her hard work has paid off with a first singles success at the age of 25.
"It's been an amazing week here in Portoroz," she said in her on-court interview. "I feel just very happy, it's a big achievement for me, to win my first title. It's special to do it on the hard court.
"It wasn't easy to go through to the final. I'm proud of myself because I never gave up.
"I'm working really hard in the past couple of months. I understood more about my game, I think. I just want to keep this level for all the season that is almost finished, and to stay strong and keep this level."