Slimani has been at the Dublin-based outfit since 2024 and won the last of his 59 Test caps during July's tour of New Zealand.
The 36-year-old will compete with ex-England tighthead Kyle Sinckler and Georgia forward Beka Gigashvili for the starting No. 3 shirt at the three-time Champions Cup winners.
Slimani's contract at Stade Mayol hands the former Stade Francais front-rower a chance to fight for a place in Les Bleus' squad at the 2027 Rugby World Cup.
"I arrive with a lot of desire," Slimani said in a Toulon statement.
"To bring my experience to the squad and to contribute to the team's performance," he added.
Slimani was once considered as one of the world's leading tight-head props, earning a reported 600,000 euros (£523k) a season during a seven-year spell at Clermont between 2017 and 2024.