You won’t find Jerry Seinfeld’s latest project on television or in theaters. Instead, try the Web.

The comedian is starring with an animated Superman in a five-minute commercial for American Express. The comedic “webisode,” which was directed by Barry Levinson and is available only online, shows Seinfeld and Superman hanging out — having lunch at a diner, hooking up a DVD player, seeing a Broadway show.

It also marks another entry in a growing niche of the marketing world: entertainment advertising, often referred to as “advertainment.” Instead of a traditional commercial, marketers attempt to disguise their message by associating their brands with other forms of entertainment.

In this case, a short film documents the mundane adventures of Seinfeld and Superman, who happen to be pals. American Express makes just a brief appearance: Seinfeld is able to exchange his brand-new DVD player — which had been damaged in a robbery foiled by Superman — because he used the card to buy the appliance.

“We can really tell a more in-depth story than we can in a short commercial, but still it’s only five minutes,” Seinfeld said at a news conference this week in Manhattan. “It seems like a fun size to make things in. A fun size, like the Snickers that come in a fun size. The big one is too much work. This is a fun size.”

The American Express online spots are similar to the short films BMW commissioned three years ago. The series of short films starred Clive Owen (“Beyond Borders”) as a driver-for-hire whose vehicle of choice is always, of course, a BMW. Although the films consisted mainly of action sequences starring the company’s cars, BMW was able to lure such big-name talent as directors Ang Lee, Tony Scott and John Woo and actors F. Murray Abraham and Gary Oldman.

Advertising experts, however, still consider such films to be a niche form of marketing, mostly because of its reliance on users having broadband Internet connections. Charlene Li, a principal analyst at Massachusetts-based Forrester Research, said that, for BMW and American Express, creating the online spots is good for their brands because their audiences are likely to have high-speed access and be willing to view the films.

Comedian Jerry Seinfeld, left, and director Barry Levinson share some laughs after the unveiling of American Express' Superman webisode at W Hotel in New York. The five-minute episode, appearing on American Express' Web site, is directed by Levinson and stars Seinfeld and Superman.