Boxer, who voted by absentee ballot a few weeks ago, delivered a victory speech about 11:20 p.m. in Hollywood.
Boxer declared the race her 11th straight victory, "and what a sweet one it was."
"From the time I was elected to this office way back, I only had one reason that I went into politics, and that was to make life better for the people I represent," Boxer told her supporters. "There's no other agenda than to make life better for you and the people of California.
"There were millions of dollars of negative ads from known and unknown opponents," Boxer said. "With 145,000 donors, we matched those special interests.
"The Giants beat the Texas Rangers and we beat the Texas polluters tonight," Boxer said to a cheering crowd.
Fiorina, who was in Irvine, did not conceded. She said the race was too close to call.
Fiorina spent $6.5 million of her own fortune to combat Boxer's prolific fundraising.
By 1:20 a.m. Wednesday, Boxer captured 51 percent of voters, about 3.2 million.
According to exit poll data, Boxer won decisively among women, pulling an estimated 56 percent compared to 39 percent for Fiorina.
The senator also was helped by voters who had a favorable view of the Democratic Party. Only 39 percent viewed the Republicans favorably.
Boxer campaigned in the Bay area earlier in the day, making a stop at an Oakland restaurant where she voiced cautious optimism.
Fiorina cast her ballot in Los Altos, near San Jose, on Tuesday morning. Fiorina said her internal polling showed the race was close, adding that she thought she had a good chance to unseat Boxer.
"I think we defined ourselves very effectively against Barbara Boxer," Fiorina said. "Barbara Boxer is a 34-year career politician who has done nothing for the people of California."
Both candidates focused on the importance of putting people back to work.
"If you want to give small businesses and family-owned businesses a helping hand so they can put people back to work, vote for Carly," said Fiorina.
"My policies are about today and tomorrow and yesterday," Boxer said. "They are about jobs. They are about made in America again. They are about middle-class people being able to walk with dignity and support their families."