The Los Angeles County coastal seat is expected to be up for grabs amid reports that Harman will resign to become president of the Woodrow Wilson International Center for Scholars later this spring.
Hahn, who represents an overlapping city council district, threw her hat in the ring this afternoon.
"I'm running for Congress to create new jobs, expand clean energy technologies and ensure that local small business owners get the help and opportunities they need to flourish in a global economy," she said in a statement.
The San Pedro Democrat, who lost a primary bid for lieutenant governor last year, ran as the democratic nominee for the seat under previous district lines in 1998.
Hahn, who has served on the city council since 2001, has high name identification in the area due in part to her family's political prominence in Los Angeles. Her father was the late Los Angeles County Supervisor Kenneth Hahn and her brother, James Hahn, was mayor of Los Angeles.
Secretary of State Debra Bowen is also considering running in the special election, which will be conducted under the state's new top two primary rules. If no candidate wins more than 50 percent of the vote in the primary, the top two candidates, regardless of party, will advance to a run-off election to fill the seat.
A special election to fill the seat will be set by Gov. Jerry Brown once the seat is vacant.
PHOTO CREDIT: Los Angeles City Councilwoman and candidate for Lieutenant Governor Janice Hahn talks with an attendee at the California Democratic Convention in Los Angeles Saturday, April 17, 2010. (AP Photo/Reed Saxon)