Sporting sign Sinama-Pongolle
In a statement, Sporting added that Atletico could receive up to a further 1 million euros depending on the player's performances.
The 25-year-old former Liverpool player joined Atletico from Recreativo Huelva in July 2008 and went on to play 37 matches for the Madrid club, coming off the bench in most of them, with five goals.
Sporting are fifth in the Portuguese Premier League, 12 points adrift of leaders Benfica and Braga. Their top goalscorer Liedson is expected to be out until late January with a knee injury.