Newsom tweeted Wednesday, "Today we welcomed the latest addition to our family. A very healthy baby girl, Brooklynn Newsom. Thank you @JenSiebelNewsom! You r amazing!"
Newsom, his then-pregnant wife and their two children rode in a convertible in Sunday's Pride parade in San Francisco.
Brooklynn joins older sister Montana, who's three, and 1-year-old big brother Hunter.