What is the capital city of Japan?

Tokyo is the capital city of Japan and serves as the political, economic, and cultural center of the country. It is one of the most populous metropolitan areas in the world, known for its modern architecture, vibrant culture, and significant historical sites. The choice of Tokyo as the capital is significant as it symbolizes Japan's rapid modernization and development since the late 19th century.

In contrast, Seoul is the capital of South Korea, Beijing is the capital of China, and Bangkok is the capital of Thailand. Each of these cities plays an important role in their respective countries but does not pertain to Japan. Thus, the correct answer is Tokyo, as it uniquely identifies the capital city of Japan.