The Clinton campaign announced Eric Holder's support in a statement to The Associated Press.
Holder, who served as President Barack Obama's top law enforcement appointee for more than five years, said Clinton "is the candidate that we need in the White House" to continue "the progress of President Obama."
The country's former top lawyer will campaign for Clinton in an upcoming swing to South Carolina.
African-American voters favor Clinton by a wide margin according to preference polls, and they will likely make up a majority of the Democratic electorate in South Carolina's Feb. 27 primary and potentially in other Southern primaries that follow in March.
South Carolina is the only one of the initial four states to vote where black voters anchor the Democratic Party base of support.
Polls in overwhelmingly white Iowa and New Hampshire show Sanders either leading or nearly deadlocked with Clinton.
Clinton has emphasized her support for tougher gun laws, trying to paint Sanders as an ally of gun manufacturers based on some of his previous votes in the Senate.
Gun violence is particularly important to black voters in South Carolina. In June, a white gunman entered a historically black church in Charleston and killed the minister and eight others, drawing a national outcry.
