A man ran onto the field and aimed a blow at the attacking midfielder, who was jogging in the opposite direction, and seemingly caught him on the neck and jaw.
Play was halted as stewards raced on to restrain the man before they led him to a nearby police officer, while players from both teams offered embraces to the 23-year-old.
Grealish last week returned from a shin injury that had sidelined him since early December and scored in a 4-0 rout of Derby County.
West Midlands Police later tweeted to confirm that a man has been arrested and is en route to a custody block "following the incident on the pitch involving an attack on an Aston Villa player".
Unbelievable scenes at #SecondCityDerby as a fan invades the pitch and throws a punch at Jack Grealish.— Heavy Hands Boxing (@_HeavyHands) March 10, 2019
Ref has ruled it an illegal knockdown and deducted a point from the fan. Grealish 10-8 round for me. pic.twitter.com/8OCxXBlGcK