His character Teddy Montgomery struggled with his sexuality throughout the teen drama's third run, before finally coming out as gay.
Donovan told TV Line that he is now "looking forward to what opportunities and adventures lie ahead".
"To be able to simultaneously take on a challenging role and bring awareness to a relevant social issue was a win-win for me," he said.
An insider on the CW drama reportedly confirmed that Donovan will reprise his role as Teddy for around five episodes next season to wrap up the character's storylines.
It was previously confirmed that Lori Loughlin (Deb) and Ryan Eggold (Ryan) will not return to 90210 next season, with co-star Shenae Grimes (Annie) criticising the manner in which both actors were written out.
Showrunner Rebecca Sinclair also stepped down from her position and has been replaced by Life Unexpected scribes Patti Carr and Lara Olsen.
The fourth season of 90210 will air in a new slot on Tuesdays at 8/7c in the fall on the CW.