The sixth-seeded American led 6-3, 4-1, but Bencic fought back to level the match at 4-all and held a break point to go up 5-4.
Riske saved the break point, though, and captured the final two games to close out the match.
"It's a huge accomplishment for me to win my first WTA title," she said. "I was here by myself, which made it even more special, just to know that I was able to do it by myself."
The 17-year-old Bencic was attempting to become the youngest winner on the WTA Tour since 15-year-old Tamira Paszek captured the Portoroz title in 2006.
The third-seeded Swiss was also playing in the first WTA final of her career.