|Family||Wife: Doris Klompus|
|First Appearance||"The Pen"|
|Last Appearance||"The Money"|
|Portrayed By||Sandy Baron|
Jack was a short tempered resident of the Del Boca Vista condos in Florida and seemed to consistently have a grudge against Morty Seinfeld (Jerry's dad). He was famous for shouting "Ahh!" while waving his hand down, which is often how he would end an argument with Morty.
He carried a Space Pen, the kind used by astronauts, which could write upside-down. He showed it around and gave it to Jerry after the comedian complimented it, leading to consternation over whether it was a gift that should have been accepted.
In “The Raincoats,” Morty asked Jack to retrieve a box of leftover Executive raincoats from his garage and send them to him in New York. Because the garage was locked, Jack broke a window to get in, cutting his hand in the process. He neglected to fix the window, which allowed burglars to enter the Seinfelds’ home and rob them.
When Jerry bought his parents a Cadillac with the money he received for a show in Atlantic City, Jack was suspicious of Morty, thinking he stole the money from Del Boca Vista’s board of directors. Morty was president of the board and Jack among others voted to impeach him. The impeachment succeeded. The Seinfelds ended up selling the Cadillac to Jack for $6,000 and tried to give the money to Jerry, who bought it back from Jack for $14,000. Jack proceeded to crash the car in Alligator Alley and Jerry, who had no travelling money left, was forced to sleep in the wrecked car. Jerry gave it back to his parents, who then left Del Boca Vista and moved into a camper they attached to the car.