Sacko, 22, joined Sporting from French top-flight side Bordeaux in 2014, but has so far been limited to playing for the Portuguese club's B team.
The former France Under-20 player joined Ligue 2 side Sochaux on loan in February and scored twice in 14 games.
He is Leeds' third signing of the transfer window, following the arrivals of Marcus Antonsson and Kyle Bartley.
Find all the latest football transfers on our dedicated page.
Leeds have signed Sporting Lisbon forward Hadi Sacko on a season-long loan with a view to a permanent deal.