Nicholson, who has spent 30 years with the city's Fire Department, was its first openly LGBTQ+ chief when Mayor London Breed chose her in 2019 to take over after the retirement of her predecessor, former chief Joanne Hayes-White.
"I am proud of the San Francisco Fire Department and the people that work day and night to protect our city and citizens. It has been a great privilege and honor to serve as your Fire Chief for the past 5 years," Nicholson said in a statement.
Breed said Nicholson "is retiring with a distinguished and decades-long record of public service" and "has shown extraordinary leadership and dedication to our great city and we are indebted to her for her service."
More details about the chief's medical issues were not released, but she is a breast cancer survivor and has been a proponent of efforts to prevent cancer among firefighters. During her career, Nicholson also suffered second-degree burns from a 2009 arson fire that injured five other firefighters.
Stay with ABC7 News for updates on this developing story.