Ron Forman, the head of the Audubon Nature Institute, planned to make the formal announcement Monday afternoon. His campaign and Landrieu's confirmed the endorsement.
Forman had about 17 percent of the overall vote Saturday, trailing Nagin's 38 percent and Landrieu's 29 percent.
Landrieu said he was pleased to get Forman's endorsement so quickly. "It needs all of its people to come together. I cannot do this by myself," Landrieu said.
A spokesman for Nagin said the mayor was not immediately available for comment.
Forman received about 17 percent of the overall vote Saturday to Nagin's 38 percent and Landrieu's 29 percent.
Forman is white, and much of his support came from Nagin's 2002 base, white conservatives. Nagin, a former cable TV executive who is black, was largely abandoned by whites this time but was supported by black voters.
If Landrieu wins the runoff May 20, he will be the first white mayor of New Orleans since his father, Moon Landrieu, held the office in the 1970s.
Nagin and Landrieu led a field of 22 candidates, which included business leaders, a lawyer and a minister. But because no one got a majority of the vote, the top two vote-getters will meet in a runoff.
Black voters made up slightly more than half of the overall vote, according to a consulting firm analyzing demographic data for the New Orleans Redevelopment Authority.
Nagin's "one shot is to get enough of the whites who liked him four years ago to like him again," said political analyst Elliott Stonecipher.
Roughly a third of the city's 297,000 eligible voters participated in Saturday's election, some traveling hundreds of miles to help decide who will lead one of the biggest urban reconstruction projects in U.S. history.
Fewer than half the city's 455,000 residents have returned since Katrina devastated the city in August; most of those still scattered around the country are black.