The midfielder, 25, has joined for 55m euros (£47m) but add-ons could increase the overall cost to 80m euros (£67.7m).His deal, which runs until 2025, also has an option to extend for a further year.
Manager Ole Gunnar Solskjaer said the midfielder was a "fantastic addition".Fernandes' move is the biggest in the Premier League so far during the January transfer. He follows Cristiano Ronaldo, Nani and Marcos Rojo in joining Manchester United from Sporting.
"For me to now play for Manchester United feels incredible," said Portuguese international Fernandes."My love of Manchester United really started when I used to watch Cristiano Ronaldo play and ever since then I have been a big fan of this great club," added Fernandes.