Tsukioka Onsen Mashu

An adults-only Niigata inn where emerald-green sulphur water fills four garden rotenburo and dinner is a Niigata kaiseki of A5 wagyu, nodoguro and Koshihikari — but only the outdoor tubs use spring water, and the private bath costs extra

Mashu opened in Tsukioka in 1968, founded by a couple who had farmed reclaimed land in post-war Hokkaido and named the inn after Lake Mashu. A 2018 renovation turned it into a 38-room 'adults' inn': no children under junior-high age, no tour groups, and no karaoke or banquet halls. The draw is Tsukioka's emerald-green water, a weakly alkaline sulphur spring that the inn says ranks first in Japan for hydrogen-sulphide ions and second for total sulphur. It's served undiluted in four garden rotenburo, one beside a koi pond edged with red Hidaka stone from Hokkaido. Know before you book: only the outdoor tubs and the paid private bath use spring water (recirculated and heated, with fresh spring water overflowing). The indoor tubs and every in-room bath, even in the suites, are heated tap water. Japanese guests rate it 4.8 across 2,536 reviews on Jalan (baths and cleanliness 4.9) and 4.69 across 2,024 on Rakuten Travel (service 4.78). Bloggers from 2023 to 2026 praised the top-floor views, the calm, the free towels waiting at the baths, and course kaiseki served in semi-private restaurant booths: A5 Niigata wagyu, nodoguro, snow crab and local Koshihikari rice with refills. Reviews from 2026 are overwhelmingly positive, and the complaints are small but specific: a slippery indoor bath floor (one blogger saw an ambulance called after a fall), black flecks floating in an outdoor tub, sticking shoji, cigarette smoke drifting from a smoking room, noise from the room next door, weak mobile signal, and dinners so large and fast-paced that one blogger couldn't keep up. Two-meal stays ran about ¥25,000–30,000+ per person in October 2026.