TVLine reports that the Smallville actor will play Tripp, a new love interest for Teddy (Trevor Donovan).
In this season's eighteenth episode, Teddy will reunite with Tripp, an old friend and former room-mate, and will shock him by confessing his sexuality.
Ritchson plays the recurring role of Arthur Curry / Aquaman on Smallville and also stars as Thad Castle in Spike comedy Blue Mountain State.
It was previously reported that Freddie Smith will make his debut as Teddy's new love interest Marco in February.
90210 returns to The CW on January 24 at 8/7c.