The diner's exterior was used for the gang's favorite coffee shop, where they often met to discuss the day's events. The hot spot also inspired the popular 1981 Suzanne Vega song "Tom's Diner."
Seinfeld, 59, and Alexander, 54, were captured entering the restaurant by Brooklyn, N.Y. resident Ali Phil.
The picture-taking fan said the duo was filming at the time of the photograph. The former costars could be taping an episode of Seinfeld's web series Comedians in Cars Getting Coffee. But fans have also been buzzing about the actor's tease to a new mystery project with Seinfeld creator Larry David.
Alexander, who played the bald-headed George Costanza on the '90s sitcom, was dressed in his character's signature casual attire while filming.
In a recent Reddit chat, Seinfeld talked about the show that put him on the map.
"The real pitch, when Larry and I went to NBC in 1988, was we want to show how a comedian gets his material," he said of Seinfeld's initial concept. "The show about nothing was just a joke in an episode many years later, and Larry and I to this day are surprised that it caught on as a way that people describe the show, because to us it's the opposite of that."