The 19-year-old scored five goals for the Scottish giants last season, but manager Neil Lennon is keen for the striker to get regular first-team football.
“I’m glad I’ve signed,” Watt said in an interview on Lierse’s official website. “It came about on Friday and all happened very fast.
“I had a meeting at Celtic and got told there was an option to come here. It’s a good opportunity and I’d like to give it a try.
“I’ve heard about the club before and I spoke to Rami Gershon (who was at Beveren) and he’s told me a lot of good things about Belgian football, so hopefully I can do well here and score some goals.”
