The One Tree Hill star will play a character named Suzanne in episode three of the new run, Zap2it reports.
The newcomer will act as a mentor for Silver (Jessica Stroup), who recently discovered that she carries the Braca gene, increasing the likelihood of breast cancer.
"Lindsey's playing a person that Silver knows from her Braca gene awareness," said exec producer Patti Carr. "She's playing a character who also has the Braca gene."
30-year-old McKeon is best known for playing Taylor James on One Tree Hill from 2005 to 2010 and also plays the recurring role of Tessa on The CW's Supernatural. Her additional TV credits include roles on House, Without a Trace and Veronica Mars.
90210 recently cast Wes Brown (True Blood) and Trai Byers (All My Children) in recurring roles for its fifth season. Justin Deeley (Austin) will depart the teen drama next season, while both Trevor Donovan (Teddy) and Lori Loughlin (Debbie) are set to return.
The series will return to The CW on October 8. In the UK, 90210 airs on E4.
> 90210 star Megalyn Echikunwoke joins CBS drama Made in Jersey
Watch a clip from the 90210 season four finale below: