In complete but unofficial returns, Jefferson — Louisiana's first black congressman since Reconstruction — received 57 percent of the vote over state Rep. Karen Carter, who had 43 percent.
Carter was unable to capitalize on a scandal that included allegations the FBI found $90,000 in bribe money in Jefferson's freezer.
In a concession speech, Carter embraced family members and pledged to work with Jefferson, especially on the area's recovery from last year's Hurricane Katrina.
"I guess the people are happy with the status-quo," she said.
Jefferson described his win as "a great moment and I thank almighty God for making it possible." He called for regional unity to focus on the hurricane recovery and in bringing back evacuees who are still scattered across the country.
The runoff election does not affect the balance of power in the House, which Democrats won control of in the Nov. 7 elections, ending the Republicans' 12-year run. The Democrats also won control of the Senate, and they will begin the new session in both chambers in January.
Jefferson was forced into the runoff against Carter when he failed to win 50 percent of the vote in a crowded open multiparty primary. Carter had sought to become the first black woman from Louisiana elected to Congress.
Jefferson, 59, was accused of taking bribes from a company seeking lucrative contracts in the Nigerian telecommunications market. He has not been charged with any crime and denies any wrongdoing.
Before the bribery scandal erupted, Jefferson had climbed to the pinnacle of the Democratic Party. He was a confidant of former President Bill Clinton and held a seat on the powerful House Ways and Means Committee.