The official, who is close to Pelosi, revealed her decision on condition of anonymity because she hadn't yet publicly announced it.
Pelosi's quarter-century of service in Congress representing a San Francisco area district in the House includes becoming the first woman in history to serve as speaker. The tea party-fueled political wave of 2010 forced the gavel from her hand to Rep. John Boehner, an Ohio Republican.
Pelosi was a major force behind the passage of President Barack Obama's health care overhaul and the Lilly Ledbetter Fair Pay Act. Even after the 2010 elections, when her party lost 63 seats, Pelosi was reelected Democratic leader by her caucus.
Pelosi's colleagues had said for days that the top leadership post was hers if she wanted it in the next Congress that begins in January. She refused to reveal her plans for a week after the Nov. 6 elections failed to give Democrats gain they wanted.
It was a crushing, but not unexpected result of a bitter year of elections that focused on the tight contests for president and control of the Senate. Throughout, Pelosi raised millions of dollars for Democratic House candidates and insisted that the 25 seat gain was within reach. But in the end, Democrats will gain at most eight seats and Republicans will keep their majority.
Waiting in the wings of Democratic ranks was Pelosi's deputy, Democratic Rep. Steny Hoyer of Maryland, the party whip, whom she has known since they were congressional interns, and North Carolina Democrat James Clyburn, assistant to the Democratic leader.
Pelosi is the daughter and sister of former Baltimore mayors. Her father, Thomas D'Alesandro, Jr., served as mayor of Baltimore for 12 years after representing the city for five terms in Congress. Her brother, Thomas D'Alesandro III, also served as mayor.