The Hoops striker has struggled for match action at Parkhead so far this season and has headed for the continent in search of regular first-team football following the arrivals of Amido Balde and Teemu Pukki.
Aberdeen were also interested in doing a deal for the 19-year-old, but Watt is happy to have signed on the dotted line with Lierse after consulting former Standard Liege and current Waasland-Beveren stopper Gershon, who spent the second half of the season at Parkhead last year.
Watt said: "I am glad I have signed. It came about (on Friday). It happened quite fast. I had a meeting with Celtic and got told that there was an option to go to Belgium and come to Lierse and I thought it was a good opportunity and I would like to give it a try.
"The negotiations went smoothly and it is all sorted now.
"I have heard about the club before but in Scotland you don't hear much about Belgian football, apart from maybe Anderlecht and Club Brugge.
"I have spoken to my mate Rami Gershon and he has told me a lot of good things about Belgian football.
"He said it was a good opportunity so hopefully I can do well and score some goals.
"I heard they haven't got an out-and-out striker that is fit just now. Hopefully I can come in and do that job and score some goals and see where it takes me.
"Hopefully I can score some goals and we can avoid relegation and go as high up the table as we can. I want to have a good season.
"Hopefully I can go and play 90 minutes for the Under-21s against Holland and see where that takes me. I will come back here and try and fight for a place for the game against Brugge."