$25k win is US Open junior champion Watson's biggest title of career
The 18-year-old from Guernsey secured her fifth straight-sets win of the week, beating top seed and former top-30 player Sania Mirza of India 6-2 6-4.
Watson's only previous senior title came at a $10,000 event in Frinton last summer.
Watson savoured the victory, saying: "I won a 10k event in Frinton around this time last year but this is definitely a step up.
"Against Sania I just told myself I had nothing to lose because she's been ranked 27 in the world before and she is a really good player. I just did a lot of running and got as many balls in court as I could."
Meanwhile, Andrey Golubev became the first Kazakhstan player to win an ATP tour title, beating third-seeded Jurgen Melzer of Austria 6-3, 7-5 in the German Open final. The 23-year-old Golubev overwhelmed the 15th-ranked Melzer with powerful shots from the baseline.