The attack on unwitting defender George Galamaz happened during a tie between Romanian clubs Petrolul Ploiesti and visiting Steaua Bucharest.
After the away side had just scored to take a 2-0 lead shortly before half-time, the home fan raced onto the pitch past seemingly lax security and made straight for Steaua defender Galamaz.
With the player waiting for play to restart and completely oblivious to the impending attack, the black-clad fan leapt into the air and punched him firmly on the side of the head. With Galamaz left in a crumpled heap, his Steaua team-mates then rushed to stop his attacker.
After bringing him down near the halfway line Steaua players exacted immediate revenge on the pitch invader, for which Serbian defender Novak Martinovic, who can be seen kicking the supporter while he is on the floor, and goalkeeper Razvan Stanca were sent off.
Finally police and security broke up the ugly scenes and dragged the hooligan off the pitch.
Play was eventually restarted but the match was later called off by the referee when a flare was thrown onto the pitch.