As dawn broke in central Paris on Wednesday, a throng of 500 people, mostly French, stood with uncharacteristic patience in a snaking line, intent on buying a decidedly un-French confection: an American doughnut. A hot, glazed Krispy Kreme doughnut, to be exact.

It was the grand opening of the chain in France, and patrons — dozens of whom had camped out overnight — watched through a giant window as a conveyor belt ferried fried dough toward a waterfall of sugary frosting. When the doors opened, they swarmed inside, ordering doughnuts by the dozen to take out, or noshing at bistro tables in a cafe-style setting.

“The French love American products, and there’s a hype around American food chains,” said Amir Boudokhane, 25, a project manager at a medical clinic, as he waited to get in. A Krispy Kreme ad had popped up on his Instagram feed, and he rushed to get there before work. “I’ll be late,” he said with a smile. “But at least I’ll have doughnuts for my colleagues.”

The sight of French people flocking to American fare might have seemed surreal a generation ago in a country that loves its Michelin-starred restaurants, three-hour dinners and iconic baguette. But today, the world’s gastronomic capital happens to be one of the biggest markets in Europe for major American fast-food chains, as evolving consumer habits, influenced by a more casual younger generation and social media, reshape the dining landscape.

In the spring, Popeye’s fried chicken drew huge crowds in Paris when it opened the first of 350 restaurants planned across France. Wendy’s has announced plans to set up shop in France. Burger King, KFC, Starbucks, Domino’s Pizza, Chipotle, Steak ’n Shake, Carl’s Jr. and Five Guys have long had toeholds, but they are rapidly expanding their footprints with plans for hundreds of new locations across the country.

Leading the pack is McDonald’s. After the United States, France is the chain’s most profitable country, with more than 6 billion euros ($6.5 billion) in sales from over 1,500 restaurants in 2022. France is also the second-biggest market for Burger King, with €1.2 billion in revenue last year.

“France is an El Dorado for American fast-food brands,” said Xavier Expilly, a consultant who helped oversee the French openings of Burger King, Five Guys and others. “Habits are shifting — people are eating faster than before, and they want a different experience,” he noted. “The American chains know how to respond to that need perfectly.”

Krispy Kreme is the latest. At the opening on Wednesday, a D.J. thumped out dance music to the waiting crowd. A red carpet adorned the entrance, while the company’s mascot — a doughnut — bobbled alongside workers handing out fresh glazed goodies to eager patrons.

Miguel Calic-Cuere spent the night in front of the store with dozens of fans eager to secure a good place in line. “I hardly slept because it was so cold,” said the 20-year-old student, who kept warm by taking short walks and making friends in the growing crowd. Waiting for the opening “was a moment of solidarity for all of us,” he exclaimed. When the doors finally opened, he found himself the lucky winner of a free box of a dozen Krispy Kremes each month for the next year.

Celia Lea Amarouche, 19, and Marie Besson, 28, were ready to open their wallets after seeing Krispy Kreme cameos in American TV shows that they had watched to improve their English, including “The Simpsons” and popular crime shows. Seeing Kylie Jenner dig into Krispy Kreme doughnuts on TikTok sealed the deal, Ms. Besson said.

“Doughnuts are little known in France compared to croissants and pain au chocolate,” said Alexandre Maizoué, the director general of Krispy Kreme France and a member of the executive committee of Wagram Finance, a French private-equity firm backing the company’s venture. He gestured around the store, where people crowded around five counters, picking out chocolate frosted doughnuts with sprinkles, or a dozen glazed in strawberry pink.

“This is all about American pop culture,” he said. Krispy Kreme “appeals to Generation Netflix,” he added. “They’ve seen all the American series. They like U.S. culture and the American art de vivre.”

Doughnuts rolled off an assembly line that had been imported from North Carolina, where the chain is based. New flavors were created to appeal to the French palate, including a fresher fruit taste for the apple doughnut and less sweetness to the strawberry frosting. One Frenchified version was loaded with gingerbread, a cookie often served with an espresso at cafes.

The store, in a premium spot that Krispy Kreme took over from the French Michelin-starred chef Alain Ducasse, will make 45,000 doughnuts a day. A dozen more sites will open in Paris in the next three months, Mr. Maizoué said, and Krispy Kreme doughnuts will be sold at dozens of French supermarkets.

Things were not always smooth for American brands in France: Burger King was forced to pull out of the French market before re-entering a decade ago, after it insisted on sticking with a burger menu that was too American for French tastes. McDonald’s, by contrast, catered some offerings to the French market by introducing add-on items not in its U.S. restaurants, like croissants and a facsimile of the croque-monsieur sandwich.

McDonald’s also set a standard by creating a sit-down restaurant atmosphere that made French consumers steeped in cafe culture feel more comfortable about gathering at a table around a Big Mac. It also uses French-sourced meats, cheeses and breads.

Other American fast-food chains have adopted that same model to survive — and thrive, Mr. Expilly said. “Sales in France are two to three times higher than in other countries because companies have modified that ultrafast American dining experience,” he said.

French food isn’t about to be knocked from its perch: The No. 1 lunch counter is still the local bakery. About 2.6 billion baguette sandwiches are sold each year in France, compared with 1.4 billion hamburgers, according to Gira Conseil, a French consultancy.

But in terms of sheer earning power, American chains dominate, making up nearly 30 percent of all fast-food sales, a broad category that includes rapid-service restaurants. The top five American fast-food companies in France had €8.6 billion in sales last year, according to Xerfi, a Paris-based research firm.

By all accounts, there is more money to be made. American chains are expanding abroad because saturation levels in the United States have reached record highs, said Aaron Allen, founder and chief strategist of Aaron Allen & Associates, a Chicago-based consultancy that focuses on the global food service and hospitality industry.

“The U.S. brands have battled it out, and there’s only so many places to put them,” he said. “So you see a rush to get to those countries where norms are changing and there’s more acceptance of fast food.”

That is the case in France, where the younger generation is more casual than its parents were and regular mealtimes at home have splintered. Hourlong sit-down lunches have morphed into shorter breaks that send people looking for a faster alternative.

And the use of food delivery services like UberEats and Deliveroo exploded during the pandemic, fueling a penchant among young people to order in with the tap of an app — a phenomenon that didn’t exist in France a decade ago.

Back at Krispy Kreme, Michele Fidel and Charlie Anglo, who work at a luxury hotel in Paris, had rushed to the opening after seeing friends post about it on Facebook. “We’d been praying that Krispy Kreme would come here!” Mr. Anglo exclaimed.

Juliette Guéron-Gabrielle contributed reporting.