Naha, Okinawa, Kerama Okinawa and Japan's southwest islands enjoy some of the country's mildest temperatures in December, with the mercury pushing the warmer side of 20°C.

In 2026, the warmest part of Japan during the winter is the Okinawa archipelago, located in the far southwest. While mainland Japan (Honshu, Hokkaido) experiences freezing temperatures and heavy snow, Okinawa maintains a subtropical climate with average daytime highs ranging from 18°C to 21°C (64°F–70°F). It is essentially the only region in Japan where you can avoid heavy winter coats, although it is still too cool for typical swimming in the ocean without a wetsuit. For those seeking "mild" rather than tropical warmth on the mainland, the southern island of Kyushu and parts of Shikoku are significantly warmer than Tokyo or Osaka, often seeing highs around 10°C to 12°C. The Izu Peninsula and the Boso Peninsula near Tokyo also benefit from the warm Kuroshio Current, offering slightly milder conditions and early plum blossoms in February, but for true warmth, Okinawa remains the definitive escape for domestic and international travelers alike.