The French midfielder has spent the last one-and-a-half seasons on loan with the Ligue 1 club, making a total of 36 appearances. He was also made captain last season.
The 25-year-old arrived on Teesside from Paris Saint Germain in 2008, making 32 league appearances for Boro.
Digard joins Kris Boyd and Andrew Taylor through the exit door at the Riverside, as Boro boss Tony Mowbray looks to raise funds to bring in his own players.
French midfielder Malaury Martin became Mowbray's first summer signing when he put pen to paper on a one-year deal following a successful trial, and the Boro boss is still keen to bring in some new faces before the Championship season gets under way on August 6.
Boro also confirmed yesterday that Argentinean Julio Arca is training with Mowbray's squad as he looks to get in shape ahead of the new season.
Arca is currently a free agent after his previous Boro contract expired at the end of last month.
Mowbray was keen to keep the versatile player at the club, but with the club trying to cut its wage bill by more than half its £20m value, Boro were unable to match the Premier League terms he was previously on.
Following their pre-season tour to Slovenia, Boro are back in action in the North-East today when they travel to The Northern Echo Arena to take on Darlington.
Striker Marvin Emnes is expected to make an appearance after recovering from a hip problem.
Defender Seb Hines could also return to action after playing in a Middlesbrough XI against Harrogate Town on Wednesday, with Mowbray keen to give all his players at least another 90 minutes before the new campaign begins.