The billionaire gave his two cents after Fiorina interjected as Sen. Rand Paul (R-KY) explained why he thought implementing a no-fly zone over Syria would be a bad idea.
"Why does she keep interrupting everybody?" Trump said. The audience laughed at first, then booed him.
Notably, the last time Trump took on the former tech executive—he yelled "Look at that face!" when he caught a glimpse of her on TV in the company of a Rolling Stone reporter—a pro-Fiorina super PAC turned the remark into a campaign video.
Trump explained the rationale behind his own interruption after the debate in the spin room.
"When somebody cuts someone else off I think it's inappropriate," Trump told moderator Neil Cavuto.
"I thought she was fine, but I thought somebody had to tell her 'It's time to stop,'" he separately told MSNBC's Kasie Hunt.
This post has been updated.