Ex-UFC star Tim Kennedy -- who's also a Green Beret -- says he's boycotting Nike in the wake of their Colin Kaepernick ad ... and says his special forces buddies feel the exact same way.
Kennedy is currently participating at a military school course at Fort Bragg in North Carolina -- where he told us he feels the ad is disrespectful to the flag and to the men and women of the U.S. Armed Forces.
"When I walked into the team room this morning, there were some people who were fuming," Kennedy tells TMZ Sports ... "I'm not gonna speak for them, but there was no one happy about it."
Kennedy's got a huge problem with the words in Colin's Nike ad -- "Believe in something. Even if it means sacrificing everything."
Tim says every member in this special forces team room has lost at least 10 friends who died while serving their country ... and he finds it offensive that Colin would describe what he's been through as "sacrificing everything."
"I'm talking about REAL heroes and I'm surrounded by a bunch of them right now."
Kennedy's not the only one boycotting Nike -- country music star John Rich says he won't buy Swoosh products and the National Association of Police Organizations has also called for its members to cut off the brand.