A Chiefs fan who stormed the field during Sunday night's playoff game at Arrowhead Stadium was absolutely rocked by Bills star Stefon Diggs -- and the wild scene was all captured on video.

The man in a Travis Kelce jersey sprinted onto the turf in the 4th quarter of the heated Buffalo vs. K.C. game ... when, suddenly, Diggs went after him.

In footage shot by a fan in the stands ... you can see the Bills wideout was so pissed over the interruption, he smashed the guy square in the back.

The hit was so hard -- even Tony Romo, who was broadcasting the game for CBS, said on-air, "Diggs took out some frustration on him."

What's crazy -- the fan actually got up after the hard hit ... but he was immediately taken down by stadium security. Authorities then rushed him off the field.

It's unclear if the fan was arrested ... we reached out to cops, but so far, no word back yet.

As for the game -- the guy probably should have stayed in his seat, 'cause he missed an all-time classic. The Chiefs went on to take down the Bills in a wild overtime finish, 42-36.