Koitogawa Onsen
Minamiboso, Chiba Prefecture, Japan
Illustration photo — not a verified picture of this exact spot.
- Location
- Minamiboso, Chiba Prefecture, Japan
- Water type
- sodium chloride
- Temperature
- 41 °C
- pH
- 7.3
Water profile
Thermicity
Sedative
LowHigh
Relaxing — deep, immediate muscle release.
Mineralisation
Restorative
LowHigh
Loaded with major ions — the essential minerals (Ca, Mg).
Buoyancy
Effortless float
LowHigh
Dense, salty water — you float without effort.
SodiumChloruresBrome
Koitogawa Onsen is a simple hot spring in Japan, taking its name from the river valley that shapes the surrounding landscape—a common feature of Japanese onsens where waterways and thermal activity intersect along volcanic geology. The soft alkaline waters of this simple spring promote relaxation and skin care, offering the accessible, restorative character that has made Japan's simple-type springs a cornerstone of everyday bathing culture. Visitors enjoy the tranquil riverside setting alongside the gentle warmth of the mineral waters.
Insider tip — Koitogawa Onsen lies near the southern tip of the Boso Peninsula, where the warm Kuroshio Current moderates the climate year-round, and the onsen mirrors this warmth with its sodium-chloride waters. The coastal setting allows bathers to hear the Pacific surf while soaking, and several facilities feature open-air baths that capture ocean breezes. Spring wildflower blooms across the Boso hills make this an attractive destination for Tokyo day-trippers.
Nearby
Frequently asked questions
How hot is the water?
The water runs quite hot here, around 41 °C — a sedative water that brings immediate muscle release.
What type of water is found here?
This spot features sodium chloride water.
What is the best season to visit?
The most favourable period for this spot is: Toute l'année (idéal automne-hiver).