Ioka, who won the vacant belt on New Year's Eve in the same Bodymaker Colosseum venue against Mexican Jose Alfredo Rodriguez, finished the contest with several brutal blows to the ribcage.
Thai southpaw Kokietgym began well in the opening two rounds but once his Japanese opponent settled into his rhythm, he was no match for the champion.
The talented Ioka, 24, improves to 12-0-0 (8 KOs) while Kokietgym, 29, is now 43-9-2 (13 KOs).
On the undercard Japanese favourite Ryo Miyazaki knocked out Carlos Velarde as he defended his WBA World minimumweight belt for the first time.
After labouring through the first four rounds, with the fighters level on the cards, he produced a stunning left hook to floor 22-year-old Mexican Velarde for good.
Miyazaki, 24, also won the vacant title - against Thai Pornsawan Porpramook - on New Year's Eve.
He is now 19-0-3 (11 KOs) while Velarde falls to 23-3-1 (10 KOs).
- Sports & Recreation