Cuban made the move official on Saturday evening during a Clinton campaign swing through Pittsburgh, in the key battleground state of Pennsylvania. At the event, Cuban did what he's done best for the better part of the last year: Take rhetorical aim at the GOP's presidential nominee. He opened the Pittsburgh rally for Clinton and her running mate Tim Kaine with a greeting for Trump—in Russian.
The billionaire's backing of Clinton is hardly surprising, given his outspoken criticism of Trump. Cuban has repeatedly blasted the GOP nominee, questioning everything from his net worth to his business acumen. Several months ago, Cuban was approached by dissident Republicans to mount an independent bid against Trump, but rejected the offer.
"Leadership is not yelling and screaming and intimidating," Cuban said. "You know what we call a person like that in Pittsburgh? A jagoff!"
For the uninitiated, Wikipedia says a jagoff "is an American English derogatory slang term ... meaning a person who is stupid or inept."
In an interview with CNN after the rally, Cuban said he thought Trump had gone "bat**** crazy," adding, ""I am ready to vote for a true leader, I am ready to vote for the American Dream. I am ready to tell the world that I am here to endorse Hillary Clinton."
Cuban first revealed his support for Clinton on Friday evening, in an exchange on Twitter with one of his followers.
The investor then made the news official at his appearance in Pittsburgh. Former president Bill Clinton, who thanked him publicly for the "ringing endorsement."
Clinton and Kaine are on a three-day tour of Pennsylvania and Ohio, after she became the first woman to accept a major U.S. political party's presidential nomination, at the Democratic convention in Philadelphia on Thursday.
- Reuters and AP contributed to this report.
Disclosure: CNBC owns the exclusive off-network cable rights to "Shark Tank," which features Mark Cuban as a judge.