Lewis is the first major black politician to go for Clinton. The New York senator is the frontrunner among the Democratic presidential contenders with Obama, the junior senator from Illinois, trailing her.
The Georgia Democrat said he believes Clinton will "reclaim our standing in the world," The Washington Times reported.
"I have looked at all the candidates, and I believe that Hillary Clinton is the best prepared to lead this country at a time when we are in desperate need of strong leadership," Lewis said.
Obama has endorsements from Jesse Jackson, Joseph Lowery and other civil rights veterans, as well as Oprah Winfrey, possibly the world's most visible black woman. Sam Riddle, who worked on Jackson's 1984 campaign, told the Times Obama does not need black endorsements because he has years as a community organizer in Chicago to give him credibility.