Kid Cudi says the First Amendment protects his decision to fire M.I.A. from his tour … making the bombshell constitutional assertion in response to her recent legal filing.

Here’s the deal … M.I.A. filed a lawsuit against Kid Cudi — claiming he unjustly fired her for making political comments during his tour despite her insisting he knew her politics when he brought her on board.

She also alleges she was fired to drum up publicity for the tour … and claims Cudi caused her to miss out on a $2.8 million guarantee from Live Nation.

Kid Cudi’s saying not so fast … because the First Amendment protects his right to dictate the artistic and political content of his show and make casting decisions consistent with those decisions.

As for any alleged lack of payment, Cudi says that’s between her and Live Nation — the company behind the business side of the tour — so she should take up her issue with them, not him.

As we reported … Kid Cudi announced M.I.A.’s firing on social media back in May — apologizing to fans for comments she made onstage that offended some people.