Chaturong Suksuk, 29, an ASEAN Para Games silver medalist, shot bride Kanchana Pachunthuek, her mother and sister at the wedding in northeastern Thailand on Saturday.
"They had an argument on private matters and Chaturong walked to his car and picked up a gun before shooting," said Matichon Wongbaokul, a police officer from northeastern Nakhon Ratchasima province.
He said Chaturong shot his bride and four others before killing himself.
One of the victims was a guest hit by a stray bullet.
A swimmer, he won two silver medals in the 2022 ASEAN Para Games in Indonesia and Cambodia.
Local media reported that he was a Thai army ranger, and had lost his right leg while on patrol on the borders.
Gun violence is common in Thailand, where guns are readily available both legally and illegally.
While mass shootings are usually rare, the attack comes months after a mall assault last month that left three dead.