Prawit, Thailand’s army chief in the early 2000s, was one of the architects of the 2014 coup that ousted Paetongtarn’s aunt, then-PM Yingluck Shinawatra.
He went on to serve as deputy prime minister under the military-backed government that ruled until last year.
The Thai parliament said in a statement that it would investigate the slapping after ThaiPBS made a formal complaint.
Prawit could potentially face suspension as an MP or even a lifetime ban if he is found guilty of a serious ethical violation.
The Thai Journalists’ Association condemned Prawit for using “violence against a journalist”.
“The Thai Journalists Association considers his actions to be threatening and harassing toward the rights and freedom of the press,” it said in a statement.
Noppadol Srihatai, the executive editor of ThaiPBS, told reporters that Prawit’s actions “threaten journalism”.
“He was seen hurting a reporter and we cannot accept it. As a public organisation, we must protect journalists’ rights so that media work won’t be affected in the future,” Noppadol said.
Prawit has apologised to the reporter, saying he “did not have any bad intention”.
