Groff appeared in the show's first season as a love interest for Rachel (Lea Michele). He later made several appearances in season two as a consultant for New Directions.
Glee creator Ryan Murphy confirmed Groff's return at a showing of the comedy's 300th musical number, according to E!.
Jesse St James will reportedly be returning as rival group Vocal Adrenaline's new coach. He will appear in four episodes, which will lead up to the season's Nationals performances.
Michele later tweeted: "So happy to have my best friend in the world Jonathan Groff back on #glee this year! Makes me the happiest girl in the world!"
Chord Overstreet will also make a return to Glee as New Directions member Sam.
> Matthew Morrison: 'I wasn't the hugest fan of season two'