The Swiss side were thrown out of the competition on September 2 after being found guilty of fielding ineligible players, meaning Celtic were handed their place in the group stages despite losing the play-offs 3-1 on aggregate.
A Swiss regional court backed Sion's fight to be re-instated in the tournament but Celtic say their opponents have lost their appeal following Tuesday's hearing.
A statement on the Hoops' official website read: "UEFA today [Tuesday] rejected FC Sion's appeal against their expulsion from the group stages of this season's Europa League.
"Celtic were reinstated to the competition, and will take on Atletico Madrid, Udinese and Rennes. The first match takes place this Thursday, September 15 at the Vicente Calderon Stadium in Madrid."