Khazri enjoyed a fruitful stint on loan with Rennes last season, scoring nine times in 27 appearances as they qualified for Europe.
That form, alongside two goals and two assists in Russia, encouraged Saint-Etienne to sign the winger on a four-year deal after paying an undisclosed fee, reported to be £6 million ($8m) rising to £9m ($12m).
The 27-year-old joined Sunderland in January 2016 from Ligue 1 side Bordeaux, having spent two seasons there following a move from Bastia. In total, he played just 42 times for the Black Cats, scoring just three goals for the club during a disappointing stay.
At 27, Wahbi has reached maturity," Paquet told the club's official website.
"He not only led his country to the finals of the World Cup but he also managed to show his great talent in a high-profile group where Tunisia faced two of the semi-finalists.
"He is a technical leader and a player of character that the club needs to keep the sporting dynamics from the second half of last season.
"The transfer window is a complex period but we must remain consistent and patient in our efforts, as we have just done to make the arrival of Wahbi Khazri happen."