The 19-year-old featured on 20 occasions and contributed five goals as Celtic won the title in Scotland last season.
However, having been told that he would be on the fringes of the starting lineup over the coming months by manager Neil Lennon, the Scottish youth international has decided to gain experience in Belgium.
"I'm glad I've signed. It came about on Friday and all happened very fast," he told Lierse's official website.
"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."
Watt scored during Celtic's 2-1 win over Barcelona in the Champions League group stages last season.