According to L.A. County Democratic Party Chairman Eric Bauman, Harman intends to resign on Tuesday morning.
The 65-year-old Democrat was first elected to the U.S. House of Representatives in 1992. Harman represents a Democratic-safe district that includes several beachfront communities in suburban Los Angeles such as Venice Beach and Redondo Beach.
Harman has been considered a moderate democrat and an expert on international affairs. Her strength has been in homeland security and terrorism issues.
Several sources with knowledge of Harman's plans say she will become the next president of the Woodrow Wilson International Center for Scholars in Washington, D.C. The center is an organization designed to bring leading experts on various issues to Washington to interact with policymakers.
Harman sent an e-mail to her constituents on Monday, telling them that heading the think tank is "a thrilling new challenge."
Harman is leaving following some well-publicized disagreements with Nancy Pelosi. Pelosi refused to appoint Harman the head of the House Intelligence Committee when Democrats won the majority in 2006. Harman had long been the ranking Democrat.
Democrats no longer are in the majority, and Harman has decided to move on, setting up a battle for her seat.
Harman was one of the wealthiest members of Congress. Her husband Sidney Harman, is the founder of Harman International Industries and the owner of Newsweek.
L.A. City Councilwoman Janice Hahn announced Monday that she would run for Harman's seat.
The Associated Press contributed to this story.