Rihanna will take on the gig despite a fourth album, "Rated R," and hit songs "Russian Roulette," "Hard" and "Rude Boy" that have a decidedly un-KCA feel. "Rated R" debuted at No. 1 on the Billboard R&B album chart.
Bieber's debut album, "My World," was released in November and went platinum in four weeks.
For more on the KCAs, which will be hosted by Kevin James, read Variety's announcement of the nominees here.