Two judges scored the bout 115-113, 115-114 in favour of Tepparith, 23, while one judge had it even at 114-114 in the Japanese city of Osaka.
"Nashiro is a strong fighter so I did the best preparation this time. If he wants to fight against me again, I'm going to accept," said Tepparith, who improved his record to 19 wins, including 11 KOs, against two defeats.
Tepparith became the interim champion in May 2011 and was promoted to full champion in November with Japanese title-holder Tomonobu Shimizu unable to defend the belt because of an eye injury.
It was Tepparith's third defence of the title. He defeated Japan's Daiki Kameda in December and then won a unification match against Shimizu by a technical knockout in April.
The 30-year-old Nashiro, ranked 14th in the WBA, said before the fight that he would quit if he lost.
"I couldn't beat him. He was really tough. I thought I won, but it's all right. I did everything that I wanted to do. I'm not changing my mind (to retire). I experienced the best career as a boxer," he said.
The defeat saw Nashiro's record reduced to 18 wins, including 12 KOs, against five defeats and a draw.