There was no knock down in the scheduled 12 rounds, but three judges scored it 115-113, 116-112, 119-110 all in favour of the Thai champion.
After a cautious first round, the two boxers fought at close range for the rest of the bout, but neither could land a decisive punch as Tepparith unleashed a series of more accurate punches.
The 23-year-old Tepparith, who became the champion after title holder Tomonobu Shimizu suffered an injury, improved his record to 19 wins, including 12 KOs against two defeats.
The 22-year-old Kameda, ranked top in the WBA, has now 22 wins, including 14 KOs, against three defeats.
Kameda is the second of three boxing brothers, all of whom have bad-boy images in the media and among boxing fans.
He was suspended from boxing for a year after his failed title bid against then World Boxing Commission flyweight champion Daisuke Naito in October 2007, for using dirty tactics including wrestling the veteran champion to the canvas.
Older brother Koki Kameda, who defended the WBA bantamweight title beating Mario Macias of Mexico by a knockout earlier in the day, was reprimanded by the Japan Boxing Commission for urging his brother to fight dirty.
Their father, Shiro Kameda, has been barred by the WBC from serving as a cornerman in its matches for abusing match officials and has been slapped with a life ban by Japan’s boxing authority.