Washington Mutual’s days in California are now officially numbered.

JPMorgan Chase & Co., which acquired the Seattle-based thrift after it was seized by federal regulators in September, announced plans to rename the 708 California branches Chase Bank by March 30.

“(Washington Mutual has) become a tainted brand,” said JPMorgan Chief Executive Jamie Dimon Wednesday in a meeting with several reporters. “JPMorgan and Chase both those brands we did the research are extremely well-known around the world. Chase isn’t that well known in California, but we’re going to change the brand; we’re going to advertise; we’re going to market.”

JPMorgan, headquartered in New York, also announced plans to open 20 additional Chase branches in California in 2009, though the company would not say where the branches would be located. The company previously said it would not close any branches in the state.

Dimon said JPMorgan will spend $375 million to refurbish existing Washington Mutual locations and open the new branches.

Washington Mutual was the second-largest financial institution in Los Angeles County as of June 30, with more than 10 percent of all local deposits. There are currently 251 branches in the greater Los Angeles area.

JPMorgan is initially rebranding just the California branches but the company plans to eventually do away with the Washington Mutual name completely.

Amid the severe financial crisis that has claimed some of the nation’s largest financial institutions, Washington Mutual suffered billion-dollar losses as a result of its heavy exposure to the subprime mortgage market. The thrift was seized by regulators Sept. 25 in what is the largest financial institution failure in U.S. history and was subsequently sold to JPMorgan for $1.9 billion.