The Glee star will be a series regular in the second season of the Starz show, Deadline reports.
His character, Ian Todd, is described as intelligent and ambitious.
Following the exit of Ezra (Martin Donovan), Ian begins to offer advice to mayor Tom Kane (Kelsey Grammer).
Groff appears on Glee on a recurring basis as Jesse St James and recently signed up for a guest role in The Good Wife.
News of his casting in Boss comes shortly after it emerged that Sanaa Lathan has joined the show as Alderman Ross's chief of staff.
Boss, which recently won Grammer a Golden Globe, will return to Starz later this year.