Ryogoku Kokugikan Stadium is Tokyo's main sumo stadium. It is located in the Ryogoku district of Sumida, an area with other sumo-related sites. There are, by the way, many of them. Most sumo wrestlers live and practice in this neighborhood.
Sumo tournaments are held three times a year, in January, May, and September. In fact, this stadium holds three of Japan's six annual sumo competitions, all of which attract huge audiences. The stadium itself can accommodate about 10,000 fans.
The Kokugikan Stadium, established in 1985, is the fourth permanent stadium built in Tokyo since 1909.
If there are no tournaments, you can see wrestlers in the training halls, the "stables" of sumo wrestlers. In the Azumazeki Stables, across the Sumida River at the JR Ryogoku Station. Watch the wrestlers' training free of charge.