In a statement released by the mayor's office, Newsom communications director Nathan Ballard said he will leave the post in February.
/*Newsom*/ called Ballard, who began as spokesman in February 2007, "unflappable, smart and a fierce advocate."
"He is a talented communicator and a consummate professional, and we will miss him," Newsom said.
Ballard has previously served as a spokesman for U.S. Sen. John Kerry, General Wesley Clark, the Democratic National Committee, the California Democratic Party and the California Labor Federation, AFL-CIO.
Ballard, 40, said he plans to spend more time with his family and to re-launch a media relations company he founded in 2004.
In the statement, Ballard praised Newsom's work on health care, environmental issues, education and equal rights.
"It has been a privilege to serve this administration," Ballard said.
There was no immediate word from the mayor's office on his replacement.
Newsom, 42, a Democrat, announced Oct. 30 that he was pulling out of the 2010 California governor's race. His second and final term as San Francisco mayor ends in January 2012.