Cuban, who once spoke fondly of Trump, also said that now Trump "scares me."
In recent weeks, Cuban met with Clinton's campaign chairman, John Podesta, to discuss a possible endorsement. An aide to Clinton said Cuban called on Thursday night, before Clinton's speech at the Democratic National Convention, to say he was in.
While campaigning with Clinton today, Cuban used the Pittsburgh slang word "jagoff" to express his dislike for Trump and said, "I’m ready to tell you that I'm ready to vote for a true leader. I'm ready to vote for the American dream. I'm ready to tell the world that I'm here to endorse Hillary Clinton."
But it wasn't that long ago that Cuban was fully backing the Republican. Trump hosted one of his largest campaign rallies last September in Dallas at the American Airlines Center, home to Cuban's Mavericks of the NBA.
The previous July, Cuban said of Trump in a statement on Cyber Dust, a private-messaging app he created, "I have to honestly say he is probably the best thing to happen to politics in a long time."
"I don't care what his actual positions are. I don't care if he says the wrong thing," Cuban continued. "He says what's on his mind. He gives honest answers rather than prepared answers. This is more important than anything any candidate has done in years."