The versatile forward will be at Elland Road for the next year and potentially longer if things go well.
The 22-year-old has already met up with his new team-mates in Ireland as they prepare for the new season.
Sacko becomes Garry Monk's third signing of the summer after Marcus Antonsson and Kyle Bartley joined last week.
Sacko spent the second half of last season on loan at French second division side Sochaux where he scored two goals in 14 appearances.
The Frenchman left his homeland to join the Portuguese giants in 2014 and notched over 50 appearances for their reserve side.
He will be available for selection for Leeds's upcoming friendlies against Shelbourne and Shamrock Rovers.