The Lake Biwa Mainichi Marathon (びわ湖毎日マラソン, Biwako Mainichi Marason) was a marathon race held in Otsu, Shiga, Japan. It was one of the prominent marathons in Japan. It was a male only competition and had IAAF Gold Label status. It was first held in 1946 and, having taken place every year since then, it was Japan's oldest annual marathon race. The early editions of the race were held in Osaka until a switch to Tokyo occurred for the 1963–64 marathons, and all subsequent races thereafter were held in Shiga Prefecture, starting in Ōtsu fronting Lake Biwa, where the race received its name. It was sponsored by Mainichi and was known simply as the Mainichi Marathon for a period. The final race was held in 2021.