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.
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 !”
Former England striker Gary Lineker branded the attack “abhorrent”, adding on Twitter: “Well done @JackGrealish1 for keeping his cool.”
West Ham midfielder Declan Rice was quick to react to the incident, tweeting: “Absolutely disgraceful what just happened to Jack Grealish”.
Brighton defender Shane Duffy added: “That is a disgrace.”
There also appeared to be angry clashes between fans before the game at St Andrew’s.
The attack on Grealish following a number of high-profile incidents of crowd trouble in Scottish football.
In Friday night’s 1-1 draw between Rangers and Hibernian at Easter Road, Gers captain James Tavernier found himself face to face with a fan who had jumped out of the home support before being led away. Police later confirmed a 21-year-old man had been arrested.
That incident came six days after a glass bottle was thrown from the same section of the stadium as Celtic’s Scott Sinclair prepared to take a corner in a William Hill Scottish Cup game and on top of a series of missile-throwing incidents this season.
Stoke’s former Scotland midfielder Charlie Adam tweeted: “Twice in three days we’ve seen players attacked by supporters, what is going on? Football needs to stamp this out quickly. Only a matter of time before someone is seriously hurt.”