KSI vs. Fournier is now officially a "no contest."

Of course, the two met in the ring at the Misfits Boxing event at the OVO Arena Wembley ... where each fighter traded punches for two rounds before KSI landed the final blow to Fournier's jaw.

At first, it looked like KSI landed a clean punch ... but upon further review, it was clear the ref missed the illegal hit -- an elbow.

Many were furious with the result ... as KSI would certainly not have been declared the winner if the official noticed what went down.

The fight was Saturday night, and by Monday, Joe had already appealed to the Professional Boxing Association. After reviewing video from the fight, the PBA ultimately decided a no contest was appropriate.

The PBA noted the elbow was not thrown on purpose ... and was "accidental."

Fournier, as you'd expect, was thrilled with the decision ... and made a video reacting to the news online.

As for KSI, he sarcastically congratulated Fournier ... and posted a few memes for good measure.