The incident occurred in the 10th minute of Aston Villa's game at local rivals Birmingham City, in the second tier of English football.
The perpetrator ran from the stands that holds the Birmingham supporters before punching Aston Villa player Jack Grealish in the head from behind.
Stewards eventually caught up with the assailant and escorted him away from the field of play.
Both sets of players reacted angrily to the incident but the game was resumed just minutes after. Grealish stayed on the pitch to score the winning goal.
Following the match, both clubs released statements condemning the assault with Birmingham issuing an apology to Grealish.
A spokesperson for the English Football League (EFL) said: "The EFL condemns the mindless actions of the individual who encroached on to the pitch at St. Andrews on Sunday afternoon. It's a situation no player should ever be faced with.
"In all circumstances the playing surface is for players, not supporters and those playing in the game must be able to do so safe in the knowledge they will not be subjected to this type of behavior.
"Whilst this incident falls within the remit of the Football Association, we will work with all the relevant parties to address the issue of player and match officials safety on the pitch and ensure the appropriate action is taken."
CNN's Wayne Sterling contributed to this report.