The 30-year-old Democrat announced his resignation on Wednesday, citing unspecified "persistent health issues."
His resignation will be effective Dec. 31.
Ridley-Thomas is the son of Los Angeles County Supervisor Mark Ridley-Thomas and represents areas on the city's Westside.
He released a brief statement saying he recently had surgery for the fifth time this year, and though he expects to make a full recovery, doctors have advised him that he'll need an extended recovery period.
Here is Assemblyman Sebastian Ridley-Thomas' full statement:
While it has been a great honor to serve as a member of the California State Assembly for the past four years, I have decided to resign from my post effective December 31, 2017.
The reason for this difficult decision is that I am facing persistent health issues. On December 18th, I underwent surgery for the fifth time this year. Although I expect a full recovery, my physicians advise that I will need an extended period of time to recuperate.
Accordingly, I submitted my letter of resignation to Assembly Speaker Anthony Rendon late yesterday. I did so with great reluctance, but even greater respect for the office, which involves constant travel, an unrelenting schedule and high stress. Unfortunately, at this point, I am no longer able to deliver the effort my constituents in the 54th District deserve. Consequently, I am also terminating my reelection campaign.
When I resume public life, I intend to remain active in civic affairs, where my passion lies. Following recuperation, I will return to the work of political empowerment, millennial civic engagement, and inspiring the next generation of leaders.
I am grateful to have worked with some incredible colleagues -- men and women of strong conscience and determination to keep California moving forward. My hope is that, in this next chapter of public life, I can help ensure that African-Americans, millennials, and all Angelenos have a strong and vibrant voice in shaping the future of the Golden State.
Supervisor Mark Ridley-Thomas released the following statement regarding his son's resignation:
Our family is dedicated to public service, and Avis and I are extremely proud of how our son, Sebastian, has represented California's 54th State Assembly District and served as a role model for the greater African American community and his generation.
Yet we, more than anyone, have seen him struggle with health challenges this year, and we fully support his decision to step down from the state Legislature so that he can recuperate with complete rest, in accordance with his doctor's orders.
City News Service contributed to this report.