The Omicron variant is triggering a fresh patchwork of national travel restrictions around the world, throwing up new obstacles to overseas travel just as it was starting to bounce back from last year’s Covid-19 measures.

The U.S. is toughening up testing requirements for inbound travelers, including U.S. citizens. Japan and Israel are barring almost all foreign visitors for now. Countries across Europe and Asia have instituted new rules for international travelers—including additional testing and quarantine requirements.

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The Omicron variant is triggering a fresh patchwork of national travel restrictions around the world, throwing up new obstacles to overseas travel just as it was starting to bounce back from last year’s Covid-19 measures.

The U.S. is toughening up testing requirements for inbound travelers, including U.S. citizens. Japan and Israel are barring almost all foreign visitors for now. Countries across Europe and Asia have instituted new rules for international travelers—including additional testing and quarantine requirements.

Many countries quickly banned or restricted travelers from southern Africa, where the new variant was first identified. But governments are also tightening up restrictions for arrivals from elsewhere as Omicron cases pop up around the world.

While mostly short of outright bans, the new rules make trips harder to plan and more expensive if additional testing is required. Quarantine rules can make business travel, in particular, impractical. Ahead of the holiday season, travelers are also weighing up the risk of getting caught mid-trip by new, more stringent rules.

South Korea said it would start requiring overseas visitors to quarantine for 10 days. In response, a United Nations peacekeeping conference in Seoul that was expected to draw more than 700 people next week will now be held online, officials said.

The CDC said the first known U.S. case of the Omicron variant has been identified in California. The coronavirus strain is spreading across the world as scientists race to find out more about its effects. WSJ’s Brianna Abbott explains what could be next for the U.S. Photo: Spencer Platt/Getty Images The Wall Street Journal Interactive Edition

The country’s semiofficial Yonhap News Agency reported the Korean boy band BTS, which is in Los Angeles for its first in-person concert in two years, won’t be able to attend a music awards event scheduled in Seoul next week because of the quarantine requirement. The band had planned to return to South Korea after its last performance in the U.S. on Friday. That is the day the new South Korean rules come into effect. A representative for the band’s agent couldn’t immediately be reached for comment.

Switzerland is requiring visitors from more than 20 countries to quarantine for 10 days upon arrival. U.S. visitors can still travel quarantine-free, for now, but those from the U.K. and several other European countries can’t. Executives say that has scrambled travel plans for bankers who have recently started shuttling again between financial capitals such as London, Zurich and Geneva.

Switzerland also hosts the World Economic Forum, a conference of world and business leaders held in Davos every January. The meeting wasn’t held last year. Organizers didn’t respond to requests for comment about how the ban might affect the event next month.

JPMorgan Chase & Co. canceled its annual festive carols reception in London, scheduled for next week, according to a company email. In past years, the event was held in a historic hall on the banks of the River Thames where top bankers met with favored clients over canapés and mulled wine.

Travelers pass through Paris Charles de Gaulle airport, where France’s new nationwide rules mean arrivals from outside the EU must provide a negative test.

Photo: SARAH MEYSSONNIER/REUTERS

In parts of Asia, the new travel restrictions are raising again the barriers that made overseas travel nearly impossible across much of the region last year. The figure skating final competition of the International Skating Union Grand Prix, which had been scheduled for early December in Osaka, Japan, was canceled because foreign skaters couldn’t come, the Japan Skating Federation said Thursday.

In Europe, the travel restrictions aren’t as draconian, though in many countries they have been tightened. For instance, France is now requiring a negative Covid-19 test for all travelers arriving from outside the European Union, including those who are fully vaccinated. Previously, people vaccinated with an EU-approved vaccine didn’t need a preflight test.

Airline booking data for international travel aren’t yet widely available for the days following Friday’s disclosure of the Omicron variant by South Africa. But early data on hotel booking in Europe and Asia show reservations are falling again, reversing some earlier gains, particularly in Europe.

For the week ended Nov. 28, bookings for hotels across Europe were down 21% compared with the same period in 2019, according to YipitData. As recently as late October, hotel bookings were 2% higher than the same 2019 period. Apart from worry about the Omicron variant, the steady decline corresponds with rising Covid-19 cases across much of the continent, attributed to the Delta variant.

The U.S. opened its borders to vaccinated Europeans and others in early November, ending a ban on most nonresident foreigners that was put in place in March 2020. That gave a big lift to the international travel industry, which has been struggling to come back since the start of the pandemic. The countries formerly on the banned list accounted for 53% of all overseas visitors to the U.S. in 2019, according to the U.S. Travel Association.

The Biden administration isn’t reinstating such a ban, but is introducing new rules for international travelers coming to the U.S. The changes aren’t as stringent as some of those made in recent days in Asia and Europe. The new rules will require travelers to test within a day of departure, regardless of vaccination status, rather than the 72 hours currently required for vaccinated travelers, senior administration officials said. The new testing rules will apply both to U.S. citizens and foreign nationals entering the country.

—Patricia Kowsmann in Frankfurt, Rhiannon Hoyle in Sydney and Shan Li in New Delhi contributed to this article.

Write to Dasl Yoon at dasl.yoon@wsj.com, Eric Sylvers at eric.sylvers@wsj.com and Miho Inada at miho.inada@wsj.com