Toshimaen Niwa-no-yu is a beloved onsen and spa complex in Koyama, Nerima-ku, set within a serene 1,200-tsubo traditional Japanese garden that instantly transports guests away from the bustle of Tokyo. The hot spring water is sourced from 1,445 metres underground and is rich in sodium, calcium, and iron, prized for its muscle-soothing and joint-relieving properties. An impressive variety of bathing experiences awaits, including carbonated water baths, sulfur rock pools, micro-bubble tubs, Shigaraki-ware single baths, Ne-yu reclining baths, cold plunge pools, and an outdoor Finnish sauna housing 600 kg of sauna stones with an automatic loyly steam system. Wellness services such as massages and body treatments are available alongside relaxation lounges, and the venue maintains an adults-only policy for an atmosphere of undisturbed calm. Accessible via Toshimaen Station on the Seibu Toshima and Oedo lines, it stands as one of Tokyo's most comprehensive natural bathing destinations.