The 19-year old striker will spend the 2013/14 season with the Antwerp outfit and is looking forward to furthering his development in the Jupiler Pro League.
"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 (ex-Celtic loanee) 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."