Ryukyu Onsen Ryujin-no-Yu (Senagajima Hotel)

Ryujin-no-Yu is the hot-spring bathhouse of the Ryukyu Onsen Senagajima Hotel, standing on Senaga Island in Tomigusuku City, about 1.5 km south of Naha Airport. It opened in February 2013. Senaga Island appears in Ryukyuan creation lore and, before the war, was revered as a sacred place for fertility prayers, dotted with the 'child-bearing rock' (Ishiirii) and various worship sites and sacred springs that drew pilgrims from the surrounding region. After the Battle of Okinawa the entire island was requisitioned by the U.S. military and residents were forced to resettle on the opposite shore. The island's worship sites and sacred springs were consolidated and relocated to Akasachi Forest across the water, becoming today's Senaga distant-worship site (finalized in 1975). The island was returned in 1977 and later developed as a resort. The hotel grounds are not themselves the sacred site, but Ryujin-no-Yu is nicknamed the 'Bath of Fertility' in a nod to that heritage.

The spring is a sodium-chloride strong salt spring, welling up at roughly 500 liters per minute from 1,000 meters underground at a source temperature of about 50C. Rich in sea-derived salts, this dense chloride water retains heat exceptionally well. Because it is a strong salt spring, the water is heated, recirculated through filtration, and chlorinated for hygiene, so it is not a free-flowing (kakenagashi) source.

The open-air area overlooks Naha Airport's second runway and the East China Sea, offering a standing bath, jar baths, rock baths and reclining baths, alongside indoor baths, a loyly sauna, a mist sauna and a cold-water bath. Day-trip bathing costs 2,000 yen for adults and 1,000 yen for children, open 6:00-24:00 (last entry 23:00), year-round. Hotel guests bathe free of charge during their stay.