Tamagawa Onsen

Published: Aug 11, 2026

Updated: Aug 11, 2026

Tamagawa Onsenacidic springgeothermal therapyHachimantaiSemboku

Representative image — not a photograph of this location.

Tamagawa produces nine thousand litres a minute from a single source at 98 degrees and pH around 1.2 — the largest single-source output in Japan and the most strongly acidic hot spring in the country. The water runs from the vent as a three-metre-wide river of steaming acid called Ozuka, and the baths dilute it because undiluted it is not survivable for long.

The other thing people come for is the ground. The bedrock around the vent contains hokutolite, a radium-bearing mineral, and the surface is warm. People lay mats directly on it and lie there for hours — a form of geothermal therapy that has drawn long-stay visitors for centuries. The site is a working cure resort rather than a leisure onsen, with a self-catering wing alongside the inn, and it sits in the mountains at the western end of Hachimantai.

Essential Information

Opening Hours

Tamagawa Onsen day bathing 10:00 AM – 3:00 PM

Closed: Access road closes in winter — confirm before travelling

Entrance Fee

Tamagawa Onsen around ¥800 adults, ¥400 children | Shin-Tamagawa Onsen ¥1,000 adults, ¥500 children from 1 April 2026

Best Season

Late April–October, while the mountain road is open

Visit Duration

2–3 hours | Long stays are the traditional use

Getting There

Access Information

Tamagawa, Tazawako, Semboku, in the Hachimantai mountains. Car recommended; the access road closes in winter. Day bathing at Tamagawa Onsen costs around ¥800 for adults and ¥400 for children, 10:00 AM – 3:00 PM. The neighbouring Shin-Tamagawa Onsen charges ¥1,000 for adults and ¥500 for children from 1 April 2026. The geothermal ground area is open to visitors, who bring or hire mats. Confirm winter access before travelling.

Insider Guide

Premium

**The bath is diluted, and that is not squeamishness.** pH 1.2 undiluted will damage skin and eyes. Follow the posted soaking times rather than treating it as an ordinary onsen. **Bring a mat for the ground.** The geothermal therapy area is bare warm rock; people lay mats and lie on it for hours. Mats can be hired on site. **Two different places with similar names.** Tamagawa Onsen is the old cure resort; Shin-Tamagawa Onsen is the newer, more comfortable hotel nearby with the same water. Prices and facilities differ. **Day bathing is a five-hour window.** Ten to three at Tamagawa. This is not somewhere to arrive late. **The road closes in winter.** Deep snow shuts the mountain access for months. Check before setting out from November onward.

View on Google Maps