The 18-year-old has risen to prominence since breaking into United's first-team in 2012, and has courted attention from several teams throughout Europe.
However, it is Primeira Liga side Sporting who have won the race for the Scot's signature for an undisclosed fee, and he will remain at Estadio Jose Alvalade until at least 2020.
"Sporting Club de Portugal and Dundee United have reached an agreement for the player Ryan Gauld, representing Sporting in the next six seasons," read a statement on Sporting's website.
"A termination clause in the amount of €60 million was set."
United too confirmed the deal stating: "Subject to the usual formalities, Dundee United confirm the transfer of Ryan Gauld to Sporting Lisbon, for an undisclosed fee, was completed last night.
"Everyone at the club wishes Ryan sincere best wishes in his future career."
Gauld joins a team that has nurtured the likes of Luis Figo and Cristiano Ronaldo, and who will play in this season's UEFA Champions League.
