Newsom sent a tweet on Monday saying they're expecting a son this winter.
Jen and I are excited to announce that we are expecting a baby boy this winter!— Gavin Newsom (@GavinNewsom) August 31, 2015
The Newsoms already have two daughters, ages 5 and 2, and a 4-year-old son.
They've been married since July of 2008.
Newsom, who is 47 years old, has been lieutenant governor since 2011, after spending eight years as the Mayor of San Francisco.