Though he hasn’t been in the workplace for long, 26-year-old Miguel Osio has already seen it undergo a strange transformation. Men—businessmen—are carrying backpacks, not briefcases. The New York-based marketing consultant has been startled both by the sheer number of toters as well as their demographic range. “I even see a few older guys, men in their 50s and 60s, commuting, getting off the subway in their suits and ties with their black Tumi backpacks,” said Mr. Osio, who carries a brown Tumi backpack himself. A briefcase, he mused, used to confer legitimacy on its owner, but that staid symbol of corporate success seems to have lost its mojo. “I’ve worked in very traditional, conservative companies, and the formality [of a briefcase] seems impractical,” said Mr. Osio. Once, a snooty receptionist would have presumptuously redirected a backpack wearer to the mailroom, declining to buzz him in. But there’s been a shift in what constitutes a boardroom-worthy bag, and backpacks are assuredly in the game.

Enter an office elevator in any city, and you’ll spot nearly as many backpacks as tightly gripped Starbucks cups. While besuited businessmen haven’t entirely abandoned briefcases and messenger bags, the backpack is gaining ground. According to NPD, Inc., which tracks retail trends, sales of adult men’s backpacks have grown steadily in the past two years. Sales of that segment increased 5% to $864 million between August 2016 and this past August, representing 48% of the entire U.S. backpack market.

Driving that uptick, in part, is the backpack’s evolution into a higher species of bag. “Men’s backpacks have gotten more executive,” explained NPD analyst Marshal Cohen. That means fewer sad-sack shapes in cheap polyester and more finely crafted designs in sumptuous leather. “Backpacks have evolved from utilitarian and technical versions, from brands like Oakley or Victorinox, to ones that are [more upscale] in leather, or a mix of fabrication,” said Mr. Cohen.

Many are as dignified as the Swaine Adeney Brigg attaché that Sean Connery carried in 1963’s “From Russia With Love,” but unlike 007’s boxy briefcase, they won’t spritz tear gas into your adversaries’ eyes. Across the board, labels are crafting backpacks built for the C-suite: supple suede rucksacks from Brunello Cucinelli; fashionable yet functional flip-tops from Milan’s Bottega Veneta; and safari-styled leather-trimmed bags from Ghurka.

At Saks Fifth Avenue in New York, white-collar shoppers are scrutinizing backpacks in much the same way men must have studied briefcases in the 1950s and ’60s, and messenger bags in the 2000s. “The backpack has exploded as the go-to accessory in the man’s wardrobe,” said Roopal Patel, the senior vice president at Saks.