The Prez -- who's vehemently against NFL players who kneel during the National Anthem -- tells the Daily Caller he disagrees with Nike's endorsement. He says ... "I think it's a terrible message that they're sending and the purpose of them doing it, maybe there's a reason for them doing it ... but I think as far as sending a message, I think it's a terrible message and a message that shouldn't be sent. There's no reason for it."

It's interesting, though ... Trump admits Nike is a tenant in one of his NYC buildings, and says it pays him a lot of rent for the NikeTown location -- but adds, the company has the right to align itself with whomever it wants.

He adds ... "It is what this country is all about, that you have certain freedoms to do things that other people think you shouldn't do, but I personally am on a different side of it."

As we've reported, Kaepernick's ad, part of Nike's 30th anniversary of its "Just Do It" campaign -- has sparked major backlash ... including from the National Association of Police Organizations, which is calling for a Nike boycott.

On the flip side, several celebs and athletes have publicly supported the ad ... even Tom Brady.

As you'll recall ... Trump made headlines in September 2017 after saying NFL owners should fire players who kneel for the anthem, referring to them as "sons of bitches."

A year later ... the war rages on.