Schiff was one of several House Democrats who have spent several months weighing whether to enter the race. Another, Rep. Loretta Sanchez of Garden Grove, said Monday that she hoped to announce her decision by the end of the week.
In a written statement, Schiff said he had given the contest much thought before opting out.
"I am very grateful for the opportunity to serve as the Ranking Democrat on the House Permanent Select Committee on Intelligence, and feel I can continue to make a significant contribution to the nation's security in that position," he said.
"I want to thank my friends and supporters for all of their encouragement and advice, and my family for their support, and I look forward to other challenges in the future."
Schiff, first elected to Congress in 2000, would have had to give up his House seat to run for Senate.
He would have faced a tough challenge from state Atty. Gen. Kamala Harris, who for now is the only major Democrat in the race.
Republicans running for Boxer's seat include Assemblyman Rocky Chavez of Oceanside and former state party chairman Tom Del Beccaro.