PacWest Bancorp (NASDAQ: PACW) is a bank holding company based in Los Angeles, California, with one wholly owned banking subsidiary, Pacific Western Bank. Pacific Western Bank has 79 full-service branches located throughout the state of California and one branch in Durham, North Carolina. Pacific Western provides commercial banking services, including real estate, construction, and commercial loans, and comprehensive deposit and treasury management services to small and medium-sized businesses. Pacific Western offers additional products and services under the brands of its business groups, CapitalSource and Square 1 Bank. CapitalSource provides cash flow, asset-based, equipment and real estate loans and treasury management services to established middle-market businesses on a national basis. Square 1 Bank offers a comprehensive suite of financial services focused on entrepreneurial businesses and their venture capital and private equity investors, with offices located in key innovation hubs across the United States.

As of March 2015, Pacific Western had $21 billion in assets and 79 locations in California, primarily in the southern and central parts of the state. Pacific Western has primarily built by the acquisition of other banks.. PacWest Bancorp was named the best bank in United States for 2017 by Forbes.

PacWest Bancorp was formerly known as First Community Bancorp. In 2005, First Community Bancorp acquired Glendora, California-based Foothill Independent Bancorp (NASDAQ: FOOT) for $238 million. In 2006, First Community Bancorp combined the two banks it then owned, Pacific Western National Bank and First National Bank, renaming them the Pacific Western Bank. In 2008, First Community Bancorp announced its plans to reincorporate in Delaware and change its name to PacWest Bancorp.

Pacific Western Bank acquired the deposits and other assets of other banking institutions which failed during the financial crisis. In 2008, Pacific Western assumed the deposits and other assets of Security Pacific Bank of Los Angeles, CA. In 2009, Pacific Western assumed the deposits and other assets of Affinity Bank, Ventura, CA. In 2010, Pacific Western assumed the deposits and other assets of Los Padres Bank, Solvang, CA.

In July 2013, PacWest announced it would acquire the commercial bank CapitalSource for around $2.29 billion.

On March 2, 2015, an $894 million merger was announced with Square 1 Financial Inc., the parent company of Durham, North Carolina-based Square 1 Bank. The bank started in 2005 grew to 13 locations and $3.1 billion in assets. The merger would result in Pacific Western becoming California's sixth largest publicly traded bank.