Showa Retro Sento Tamagawa Onsen is a day-trip natural hot spring facility located in Tokigawa Town, Hiki District, Saitama Prefecture. Originally opened in 1993 as Tamagawa Onsen Sanatorium alongside a golf course operated by the Ryokuei Group, the facility was taken over by Onsen Dojo Co., Ltd. in 2011 and rebranded with a Showa-era public bathhouse concept. Despite being only about one hour by car from central Tokyo, it is surrounded by the peaceful satoyama countryside. The facility celebrated its 30th anniversary in November 2023.

The spring water is classified as an alkaline simple hot spring with a remarkably high pH of 10.1, one of the highest in Japan. The source water rises from the Chichibu Paleozoic stratum at a depth of 1,700 meters with a temperature of 26.8 degrees Celsius and is heated before use. No additional water is added, though filtration and circulation are employed. The water is renowned for making the skin remarkably smooth and silky, earning its reputation as a beauty spring.

The facility features two bathing areas that alternate weekly between men and women: Sho-no-Yu and Wa-no-Yu. Sho-no-Yu has a rock-built open-air bath offering views of the surrounding satoyama landscape. Wa-no-Yu recreates a nostalgic Showa-era downtown bathhouse, complete with painted murals of Red Fuji and Showa streetscapes, and retro advertisements for Hechima Cologne. The facility also houses the Tamagawa Dining Hall and bodycare services.