Seeley revealed the news in a post on his website, adding that he will appear in several instalments of the show.
"So, looks like I'll be joining the cast of 90210 for the first few episodes of season four!" he wrote. "Will post more info and pics soon.
"Tell everyone you know to watch, because there'll be some drama for you and your momma!"
Seeley recently starred in the comedy Glory Daze, which was dropped by TNT earlier this year.
He has also had appearances in shows including Dawson's Creek, One Tree Hill, The Suite Life of Zack and Cody and The Closer.
Justin Deeley, Megalyn Echikunwoke and Kristina Apgar are among the other stars who have signed up for roles in the new season of 90210.