Fourth-seeded Stearns, who played two years of collegiate tennis for the University of Texas at Austin, needed two hours and 22 minutes to get past Townsend, a former doubles world number one who was playing in her first WTA singles final.
Townsend broke Stearns to open the match, but she was unable to convert a pair of set points in the ninth game and was broken in the 10th as they went to the tiebreaker.
Townsend fended off four set points and had one more of her own before Stearns pocketed the first set.
They were all square at 4-4 in the second after an exchange of four breaks, but Stearns gained a decisive break for a 6-5 lead and calmly served it out.
The American, ranked 62nd in the world, added the hardcourt Austin title to the claycourt trophy she lifted in Rabat in 2024.
