Former Mayor Mike Bloomberg, who described Donald Trump as a “dangerous demagogue” at last summer’s Democratic National Convention, spoke on the phone with the president-elect Monday, a source confirmed to The Post.

The conversation between the two billionaires was the first since Trump’s election.

Bloomberg was a supporter of Hillary Clinton during the presidential campaign, and delivered one of the harshest attacks against Trump at the Philadelphia convention.

“There are times when I disagree with Hillary Clinton. But let me tell you, whatever our disagreements may be, I’ve come here to say: We must put them aside for the good of our country. And we must unite around the candidate who can defeat a dangerous demagogue,” Bloomberg said.

Trump responded on Twitter by taking a poke at the former mayor’s size, dismissing him as “little Michael Bloomberg.”

Bloomberg aides declined to comment on the conversation.