Obama is vying to be the nation's first black president.
Byrd's support comes almost a week after the Illinois senator's 41-point loss to Hillary Rodham Clinton in the longtime lawmaker's home state of West Virginia.
Byrd said he had no intention of getting involved while his state was in the midst of a primary. ''But the stakes this November could not be higher,'' he said in a written statement.
Byrd said Obama has the qualities to end the Iraq war, which he has strongly opposed.
''I believe that Barack Obama is a shining young statesman, who possesses the personal temperament and courage necessary to extricate our country from this costly misadventure in Iraq, and to lead our nation at this challenging time in history,'' Byrd said.
Byrd has repeatedly apologized for his time in the Ku Klux Klan, which he joined as a young man in the 1940s to fight communism. He also opposed integrating the military, and filibustered the 1964 Civil Rights Act.
Byrd is the longest-serving senator in history. As Senate president pro tempore, he is in line for the presidency after the vice president and House speaker.
He is the third West Virginia superdelegate to endorse Obama, joining fellow Sen. Jay Rockefeller and U.S. Rep. Nick Rahall.
Superdelegates are party leaders who can cast individual votes for presidential candidates at the Democratic Party's convention. West Virginia has 10 superdelegates, three of whom have endorsed Obama and three of whom have endorsed Clinton.
The three who endorsed Clinton are Belinda Biafore, Pat Maroney and Marie Prezioso, party leaders with largely behind-the-scenes roles.
In the wake of Byrd's endorsement, the state's other undecided superdelegates said they are still making up their minds.
Gov. Joe Manchin, U.S. Rep. Alan Mollohan and state party Chairman Nick Casey said they remain undecided. Alice Travis Germond, secretary to the Democratic National Committee, will be involved in counting the delegate votes at the party's convention, and has said she will remain neutral to avoid any appearance of favoritism.
''It's an interesting situation, because the results seem to be going one way nationally, but here in West Virginia it went a different way,'' Casey said.
Republicans, who hope to keep West Virginia's five electoral votes in November, scoffed at the endorsement Monday.
''Even though it is apparent that Barack Obama and Senator Byrd share the same tax-and-spend values, it is surprising that Senator Byrd would endorse a candidate whom West Virginia voters so obviously rejected the week before,'' said Katie Wright, a spokeswoman for the Republican National Committee.