The incident occurred less than 10 minutes into the game when Grealish was in the Birmingham penalty area and a spectator in a flat cap ran on to the field and threw a punch from behind the visiting captain, striking him on the side of the head.
The intruder was quickly restrained by stewards as several other players rushed in to confront him. He was led away by police, blowing kisses to the crowd as he left.
West Midlands Police confirmed a man had been arrested following the incident.
Grealish appeared unhurt and was comforted by team-mates and opponents, later scoring the winner as Villa triumphed 1-0 at St Andrew’s.
And Chelsea loanee Hector posted on Twitter: “All about creating a good atmosphere in a derby game.
“But this right here is a disgraceful, embarrassing and cowardly behaviour from one joke of a man.”
Birmingham condemned the attack as "deplorable and disgusting" and Press Association Sport understands the club will apologise to Grealish personally.
It is also believed Blues will take the strongest possible action and ban the man for life from St Andrew's.
The English Football League (EFL) condemned "the mindless actions of the individual" involved and pledged to work with the Football Association to "ensure the appropriate action is taken".
An EFL spokesman said: "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 behaviour.
"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."
The game was televised live on Sky Sports Football and former Villa midfielder Lee Hendrie, on duty as a pundit during half-time, called for a severe punishment and suggested the game would have had to be abandoned had Grealish suffered a serious injury.
Hendrie said: "I'm all for the rivalry, I'm all for the banter, but that is literally the worst thing I've seen on a football pitch.
"Jack Grealish, one of the key players - what if he breaks his jaw and has to go off the pitch? That's one key player who is out of your side. If that would have happened, I think the game would have had to have been abandoned.
"A fan running on to the pitch and taking someone like Jack out, that could have been anything, there's knife crime... anything could have happened out there."
Former Manchester United defender Gary Neville called on the authorities to throw the book at Birmingham, tweeting: "The club are going have to take a huge punishment for this to act as a deterrent in the future . A Points deduction or Empty stadium for 10 games !"
There also appeared to be angry clashes between fans before the game at St Andrew's.