SAN FRANCISCO — It looks like San Francisco is about to lose its most eligible bachelor.

Mayor Gavin Newsom, 40, proposed to his girlfriend, actress Jennifer Siebel, 33, while the couple was vacationing over the weekend in Hawaii.

A wedding date has not been set, said Stanlee Gatti, a personal friend and professional party planner who spoke with the pair after the mayor popped the question.

It will be Siebel’s first marriage and the second for Newsom, whose four-year marriage to legal analyst Kimberly Guilfoyle ended in divorce in March 2006.

The two were introduced by mutual friends in October 2006 and have been a steady item ever since. Siebel, who lives in Los Angeles, stayed close to Newsom’s side in February after he publicly admitted having had an affair with a staff member’s wife and a drinking problem.

“I actually think they were made for one another,” Gatti said. “You know the kind of thing where people finish each other’s sentences? They do. Yet they’re respectful of each other.”

A Marin County native and Stanford University graduate, Siebel, 33, currently has a recurring guest spot on the NBC police drama “Life.” She also appears in the film “In the Valley of Elah” (2007), an Iraq War drama costarring Tommy Lee Jones and Charlize Theron.

In an interview published in the Nob Hill Gazette in November, Siebel was asked if she and Newsom ever discussed about what life would be like if she were married to the mayor.

“People do mention the ‘first lady’ thing to me and it makes me sort of shy,” Siebel said. “But yes, we’ve talked about it. And joked about it, too. We both care about creating a normal, balanced life that includes family.”