At men’s apparel shows this spring, runway models sported a look that many fashion experts believe is just wrong. “It’s not acceptable,” says Sam Spector, a fashion stylist whose clients include actors Daniel Radcliffe and Neil Patrick Harris.

Wearing socks with sandals—a source of endless ridicule for dads, German tourists and hippies—is allegedly in vogue. Calvin Klein Collection, Bottega Veneta, Marni and other luxury labels showcased the pairing at men’s runway shows in June. Attendees at men’s New York Fashion Week in July wore the comfortable combination. Footwear-maker Teva recently started selling sock-and-sandal packages while offering an online guide on how to pull off the look.

Part of the credit, or blame, for the trend goes to the sports locker room. One recent afternoon before practice, about a dozen New York Giants players trotted out of the locker room to speak to the media. Half of them wore socks with sandals.

“Everybody that I know wears socks with flip-flops, except females,” says Brandon Meriweather, an eight-year National Football League veteran who doesn’t limit that look to the locker room. When he shows up at the mall wearing dark sandals with white socks, Mr. Meriweather says strangers ask two questions: “Are you Brandon Meriweather?” and “Why are you wearing socks with flip-flops?”

Mr. Meriweather and other athletes say they wear socks with sandals because they can. The locker room has no dress code, and their feet take a beating in cramped cleats. It’s nice to relax their toes.