Set 168 for victory, the Stars could only manage 143 for five in reply as they slipped to a second consecutive defeat after losing to the Cape Town Blitz on Friday.
Reeza Hendricks starred for the Jozi side with an attractive 62 off 58 balls that included five fours. Captain Temba Bavuma made 33 in support.
But for the remainder of the batting order it was an average day at the office as the Giants bowlers took control in middle to late overs.
Chris Morris was the leading Giants bowler taking one for 20 in his four overs while Imran Tahir claimed one for 24.
NELSON MANDELA BAY GIANTS: Imran Tahir, Jason Roy, Chris Morris, Jon-Jon Smuts (capt), Junior Dala, Farhaan Behardien, Matthew Breetzke, Onke Nyaku, Ben Dunk (wk), Heino Kuhn, Nandre Burger.
JOZI STARS: Chris Gayle, Reeza Hendricks, Rassie van der Dussen, Temba Bavuma (capt), Dan Christian, Kagiso Rabada, Ryan Rickelton (wk), Dane Paterson, Simon Harmer, Aaron Phangiso, Delano Potgieter.