The most popular sports in Japan are sumo Wrestling (also considered a martial art), Martial Arts such as karate, judo, kendo, kyudo, and aikido, soccer, baseball, and Basketball.
Sumo is not so popular now, most young people don't watch it and have no idea who the current champion is. And many of the older generation have lost interest as the yokozuna (grand champion) is never Japanese anymore. (amateur's answer)
Baseball is the most popular sport in Japan.