Stosur headed into the WTA International competition as favourite having previously triumphed in 2009 and 2013.
And the 2011 US open champion did not disappoint in Osaka, scooping the trophy for the third time - and her sixth WTA crown overall - by overcoming Diyas 7-6 (9-7) 6-3.
Despite appearing in her first Tour final, Diyas showed few nerves as she began in fine style.
The 20-year-old broke her opponent in the first game and appeared set to take the lead when moving 5-3 ahead.
But Stosur found her best when needed, levelling at 5-5 before eventually taking an entertaining tie-break, in which Diyas twice held set points.
There was no such drama in the second set, however, as the Australian's experience told.
Stosur broke Diyas in the very first game of the second set to assert her authority, and never looked back as she sealed her victory by breaking again in the ninth game of the set.