US Open champion Stephens prevailed in the tie-break to win it 7-6 (7-5) 6-1 in an hour and 31 minutes.
In the first meeting between the pair, world number five Ostapenko made some 48 unforced against Stephens' strong defence which took her away from her first win in Miami.
The 25-year-old world number 12 now has all six finals she has competed in but the recent title is special as she is the last one to win it at Key Biscayne before it moves next year to suburbia.
"This place is pretty special to me. I grew up playing tennis here. When the USTA was here, we played Orange Bowl and all sorts of tournaments here," Stephens told press post her title triumph.
"I'm definitely happy that I could be the last person to win here [in Key Biscayne]. I have had some amazing experiences here, and I'll definitely miss it. But I just feel fortunate that I was able to do that here in South Florida with all my friends and family watching."
With this victory Stephens will make her long-awaited Top 10 debut, finishing at World No.9. (ANI)