The France Under-21 international completed his move to the Groupama Stadium on Friday night for a €22m (£19.7m) fee.
Earlier in the day, the Scottish champions revealed that they had accepted an offer for Dembele, with Brendan Rodgers bidding farewell to the talented forward.
On Thursday night, Rodgers stated that he expected Dembele to remain at the club, which prompted the player to post two cryptic messages on Twitter seemingly taking aim at the Hoops boss.
"It is vital to make clear that we have never promised any player that he can leave the club at a particular time," Rodgers was quoted as saying on the Celtic website. "It has never happened. In fact, we have said all along that we did not wish to sell Moussa, given the circumstances within the transfer window and that is why we rejected a significant offer yesterday.
"However, this particular decision to accept this offer has been taken in order to serve the best interests of the first-team squad, my coaching team and the culture and environment we have created in these last two seasons. The board and myself are united and they have been very supportive to me on this issue."
The deal was first announced by Lyon president Jean-Michel Aulas in a press conference which also confirmed the arrival of Brest forward Lenny Pintor, before the club posted an official press statement on their website with more detailed information on the two signings.
Dembele made 94 appearances for Celtic, scoring 51 goals and registering 18 assists.