The top seed prevailed 7-6(7) 6-3 over her 20-year-old opponent, who was competing in her first WTA final.
The 2011 US Open winner was broken in the first game of the match as Diyas confidently held on for a 5-3 advantage before losing serve for the first time as she tried to close the set out.
Diyas recovered in the tiebreak and held two set points but could not convert as the 30-year-old Stosur fought back to take it and avoid dropping her first set of the week.
The Queensland native, ranked 18th in the world, then scored an early break at the start of the second set and claimed another to seal her sixth career title and first of the season.
World number 37 Diyas, who had never reached a WTA semi-final before, was one of four players selected earlier this week to take part at the WTA Rising Stars Invitational event on the sidelines of the WTA Tour finals in Singapore later this month.