A historic cure-bath in the hills a short way from Unzen's main street, said to have hosted Siebold and Yoshida Shōin. An octagonal wooden bathhouse with milky sulfur water cooled from a 90°C source. No open-air bath, but Unzen's finest water.
A day-bath with a simple lodging near the entrance to Unzen Jigoku. Every tub is free-flowing milky sulfur water — indoor, open-air and four private outdoor baths. ¥500, 9 am to 9 pm; an easy stop before or after touring the 'hells'.
A communal bath in Obama Onsen on Tachibana Bay, whose ~100°C springs release more heat than any in Japan. The single-storey wooden building from 1937 feels like a museum of Showa-era life and is a designated prefectural townscape asset. Pay at the old-style counter, then choose between the hot and very hot tubs of free-flowing chloride water. Open from 6 am; part of the Kyushu 88 Onsen trail. About an hour from Nagasaki city, on the way back from Unzen.
A municipal day-use onsen entered straight from the Shimabara Ichibangai shopping arcade in this spring-water castle town. Magnesium-sodium bicarbonate water (source 61.8 °C) is used free-flowing — clear, colourless and soft on the skin. The building has a retro Shōwa-era bathhouse feel, with a sauna and tatami rest hall; admission ¥530, or ¥320 for over-70s. Open 10:00–21:00 (last entry 20:30), closed Wednesdays. Within walking distance of Shimabara Castle, the samurai quarter and the carp-stream streets, and a handy stop between Shimabara port and Unzen. Parking free for two hours.
Because drilling is banned inside the national park, every bath in Unzen runs on naturally rising water. This little communal bath on the edge of the spa town, known locally as 'Dankyū-buro', has just one deep oval pool into which the acidic hydrogen-sulphide spring flows fresh; depending on the season and the day it may be milky white or nearly clear. It is hot, 43–44°C, with a faint sulphur scent. Adults ¥200, children ¥100; open 09:00–22:00, closed Tuesdays. No dryers or amenities — soap is sold — but there are lockers, and parking for four cars. After touring the Unzen Hells, a rare chance to taste the local bath for ¥200, a world apart from the big tourist bathhouses.
A large day spa where you can gaze at Nagasaki's night view — ranked among the 'new three great night views of the world' — from the bath. Built on the slope of Mt Inasa, its rooftop open-air bath looks down over Nagasaki harbour and the city, which turns into a sea of lights after sunset. The alkaline simple water fills carbonated, electric and jet baths; there are several saunas and cold plunges, six family baths with rest rooms, a suite of bedrock baths (extra charge) and a self-service canteen. Adults ¥850 on weekdays, ¥900 on weekends and holidays, children ¥450; family baths from ¥3,000 for 60 minutes. Open late, 09:30–01:00 (until 02:00 on Fridays, Saturdays and holiday eves; last entry 30 minutes before), with no fixed closing days apart from maintenance. A free shuttle bus runs from JR Nagasaki Station (about 20 minutes), a big plus without a car. The classic plan pairs it with the Inasayama ropeway: see the view from the summit, then soak. 320 parking spaces; payment is mainly cash.
Obama Onsen in Unzen, Nagasaki, is famed for sunsets over Tachibana Bay and for spring water near 100°C – the highest heat output of any onsen town in Japan. Hama-no-Yu is the city-run, old-fashioned public bath of this seaside spa: a classic sentō layout with an attendant's booth and two deep tubs, hot and tepid, brimming with alkaline simple spring water. No soap or shampoo is provided (sachets are sold) – there is nothing here but the water, and bath lovers cherish that purity. Admission is an astonishing ¥150 for ages 6 and up (Unzen residents ¥100), free under 6; open 6:00–22:00 (last entry 21:30), irregular closures. Park free at the nearby Yūhi-no-Hiroba lot (10 spaces). Along the shore are Japan's longest footbath, the 105 m 'Hot Foot 105', and steam cookers, and it is fun to bath-hop to the Wakihama public bath (¥200). About an hour from Isahaya Station by Shimabara Railway and Shimatetsu bus, or an hour by car from Nagasaki city. Taste Unzen's mountain baths and Obama's sea baths in a single day.