The 23-year-old winger joined the Whites on a season-long loan from Portuguese side Lisbon last summer as part of a deal which gave United the option to sign the player at the end of the loan spell.
Leeds have now exercised that option and the pacy Frenchman has joined the club for an undisclosed fee – believed to be between £1.5m and £2m according to reports in Portugal.
Sacko made 42 appearances for United last season, scoring twice, and the winger becomes the first permanent transfer completed by new owner and chairman Andrea Radrizzani.
Leeds United managing director Angus Kinnear said: “We are delighted to announce the signing of Hadi.
“He is an exciting player with great potential and a player we hope to see develop over the next three years.”
In a video message to the club’s fans posted on his Twitter page, Sacko said: “Thank you to the fans for last year for the beautiful atmosphere - for the support away and at home and for everything.
“I hope for next year the same and more if you can. Thank you and see you next year.”