Clinton’s campaign office was slated to make the announcement Thursday, the Los Angeles Times reported, adding Reiner also plans to throw a fund-raiser party to celebrate Clinton's 60th birthday at his Brentwood estate on Oct. 21.
Reiner, who has directed the films "The Princess Bride" and "When Harry Met Sally," made his decision after meeting with Clinton, Illinois Sen. Barack Obama, former North Carolina Sen. John Edwards and New Mexico Gov. Bill Richardson.
Reiner’s fellow Hollywood powerbroker, Steven Spielber, announced in June that he was backing Clinton.
"I've been around a long time in the political wars, and I've been on the front lines of them," Reiner told the Times. "I'm interested in someone who can really manage those political waters. Every one of the Democratic candidates is strong, but Hillary is head and shoulders above the rest."