MMA fighter Josh Neer -- who's competed for UFC and Bellator -- obliterated a local heckler at an Iowa gym ... dishing out ferocious head strikes ... and recorded the whole thing on video.
According to the 31-year-old fighter ... a heckler named Patrick Martin had been "talking s**t about MMA fighters for a month" via text messages ... so Neer called him out on it.
"I usually ignore him ... but this time I said, 'If you really want to fight come to the gym Monday at 5:30.'"
Among the harassing texts ... Martin wrote, "Lol you are a f**king joke! You will respect me though! And I hope we can be friends afterwards!"
The heckler -- who Josh says is 6'6" and 270 lbs -- accepted the offer to fight on Monday ... and as you can see on the video ... he proceeded to get his ass kicked.
After the beat down, Neer went to social media and wrote a message directed at Martin -- "Hopefully he can learn a lesson and humble himself."