The 24-year-old Ivorian winger only joined the Black Cats from Brighton this season and played 15 games, scoring one goal and providing one assist.
He joins Monaco on a reported loan fee of £870,000, with an option to buy for £16m.
A Sunderland statement read: "Sunderland AFC today confirmed the departure of Simon Adingra.
"The 24-year-old winger has joined AS Monaco on loan until the end of the 2025-26 season, subject to international clearance.
"An option to make the transfer permanent is also included in the agreement.
Black Cats Sporting Director added: "When considering the balance and priorities of the squad, we felt this was a mutually agreeable opportunity that ensures Simon plays regularly throughout the second part of the season.
"The stature of this move represents the quality and professional characteristics he possesses, in addition to his motivation to play every week. We wish Simon well and will monitor his progress closely."
Sunderland midfielder Adil Aouchiche has also left the club to join German side Schalke, but on a permanent transfer.
Alvaro Arbeloa has moved quickly to defend Vinicius Junior after the Brazilian winger was booed by sections of the Santiago Bernabeu during Real Madrid's 2-0 win over Levante, insisting the forward remains one of the best players in world football.
Robin Roefs was the hero as Sunderland ended Everton's FA Cup campaign, saving three penalties in a shootout at Hill Dickinson Stadium to send the Black Cats into the fourth round.
Thomas Frank has admitted Tottenham failed to finish the job after Spurs were held to a 1-1 draw by Sunderland at the Tottenham Hotspur Stadium on Sunday, despite a dominant first-half display.
Thomas Frank has admitted the challenge of leading Tottenham Hotspur through a difficult transition had been "hard to enjoy", but insisted he would keep pushing as he works to get the club moving in the right direction.
