The 28-year-old actor will reprise the role of Dave Karofsky in at least four of the remaining 13 episodes, according to The Hollywood Reporter.
It has also been reported that Karofsky will appear as the boyfriend of Blaine (Darren Criss).
Blaine and Kurt (Chris Colfer) were in a relationship when the fifth season concluded earlier this year, but creator Ryan Murphy has confirmed that the final season will feature a time jump.
Adler last appeared in 'Glee' in the 17th episode of last season, when he featured in a dream sequence.
'Glee's sixth and final season, consisting of 13 uninterrupted episodes, is expected to air next year.