The 18-year-old playmaker has signed a six-year deal with the Portuguese giants after they made a £3million swoop.
His switch caps a remarkable rise from the Tannadice youth ranks, and the player - dubbed "mini Messi" by his old Tangerines team-mates - voiced his appreciation to Arabs boss Jackie McNamara and the rest of the club's officials when he returned to Scotland after being paraded in Lisbon.
Gauld said: "When I first heard of Sporting's interest from the manager at United, I was a bit overwhelmed by it.
"But the club have been great about it. They have not stood in my way. They let me speak to Sporting and helped the deal go through, so I'm really thankful for that.
"The United fans have been absolutely superb with me, ever since I played my first game. They have supported me, even when I haven't been playing too well.
"I have to thank the coaches at the club who have developed me as a player, and the gaffer for giving me my chance to show what I can do."
Eight goals in 28 appearances last term as well as picking up the PFA Scotland young player of the year award nominee catapulted Gauld to national prominence.
Sporting have already demonstrated just how highly they rate the Scotland Under 21 after inserting a £47.8m release clause in his contract.
"Going abroad is what I've always wanted to do - since I was young I wanted to play abroad," he said. "Hopefully it will live up to all my expectations.
"Everyone knows Sporting is a big club which is good at developing younger players and that has played a big part in my decision obviously.
"It's been a wild few days and I'm just happy that it's all sorted now.
"I knew it is a great club which is great for developing players so it was an option I couldn't turn down.
"They have paid a big price but hopefully I can put in good performances, establish myself and do well.
"I'm going to have to work as hard as I can, I've got a lot in my game to improve.
"It will be hard to leave home and learn a new language but I'm also really looking forward to it. It is a great challenge. I'm excited.
"The 60m euros buy-out clause is a bit unbelievable! Hopefully in a few years I can live up to that.
"I've got a six-year deal and I think that shows Sporting are not expecting too much too soon, it's probably more of a long-term investment."