Robert De Niro was SUPER PISSED onstage during the Gotham Awards Monday night, claiming someone edited his speech about Donald Trump ...  but he read it anyway.

The two-time Oscar winner was introducing the Gotham Historical Icon and Creator Tribute award for his new film, "Killers of the Flower Moon," when he went off script and unleashed his mini tirade.

De Niro said, "I just want to say one thing. The beginning of my speech was edited, cut out, and I didn’t know about it. And I want to read it."

He then lectured the audience about history and truth "being replaced by "alternative facts." He also took aim at John Wayne, accusing the iconic Western actor of supporting the taking away of land from Native Americans.

De Niro finally got to the point of his speech, which was to blast Trump -- a man he passionately hates.

He said, "The former President lied to us more than 30,000 times during his four years in office. And he’s keeping up the pace in his current campaign of retribution. But with all his lies, he can’t hide his soul."

De Niro hit Trump with a few more zingers before veering off, "This is where I came in and I saw that they edited all that."

He blamed Apple and the Gotham Awards for tinkering with his words, adding, "I don’t feel like thanking them at all for what they did. How dare they do that, actually."