The endorsement comes as Jackson Jr. engaged in a contentious re-election campaign against primary challenger Debbie Halvorson.
"Thank you for your strong leadership in the Congress of the United States," Pelosi said. "That's why I am here today to endorse Jesse Jackson Jr. for re-election to the Congress."
She said he presided over the debate for President Barack Obama's health care overhaul "very, very excellently." She also praised his work in the appropriations committee.
Pelosi made the announcement at the headquarters of the Rainbow PUSH Coalition, the civil rights organization run by Jackson's father, the Rev. Jesse Jackson.
"Leader Pelosi didn't have to endorse me but she did and I'm grateful," Jackson Jr. said.
Halvorson also scheduled a news conference to respond to Pelosi's visit.
"The last time I checked, Nancy Pelosi doesn't vote in the 2nd Congressional District," Halvorson said.
Jackson has represented the 2nd District since winning a special election in 1995.
Jackson has already received endorsements from President Barack Obama and chicago Mayor Rahm Emanual. Pelosi is the highest-ranking Democrat to appear with Jackson during his re-election bid.
Jackson is under investigation by the house ethics committee over allegations that his supporters offered $1.5 million to convicted former Gov. Rod Blagojevich to get Jackson appointed to Obama's former Senate seat. Halverson, of Crete, said the probe is a distraction that prevents Jackson from effectively representing the district.
(The Associated Press contributed to this report)