A day-use-only onsen on the Otaki side of Yoro Valley on the Boso Peninsula, near Awamata Falls. A rotenburo looking over wooded hills and a timber indoor bath hold the dark-tinted alkaline water typical of Boso. Popular after hikes and waterfall walks; towel and robe rental mean you can come empty-handed. Closes early at 18:00; Wednesdays off.
About 70 minutes from central Tokyo via the Aqua-Line, a hot-spring inn founded in 1950 on the shore of Lake Kameyama in the Boso hills. Its chocolate-brown 'kuroyu' bicarbonate water — billed as Boso's finest beauty bath — flows freely into a large bath with lake and forest views. Day use 11:30–18:00, ¥1,000 on weekdays; no day bathing Thursdays and Fridays. In autumn pair it with a foliage cruise on the lake.
One of the Bōsō peninsula's largest spa resorts, in Shirako on Kujūkuri Beach. Soak in iodine-rich, tea-coloured Chōsei spring water in the open-air bath, with seven rock-bath rooms, saunas, nap lounges, banquet halls and lodging on site. Open straight through from 10:00 to 08:00 the next morning; stays past 01:00 incur a late surcharge. Free shuttle from JR Mobara Station (reservation required). ¥1,600 weekdays, ¥1,900 weekends. Tattoos—including stickers and paint—are strictly refused.
A 12-room inn in Yoro Valley, the Boso Peninsula's autumn-colour spot and the last place in the Kanto region to turn, peaking late November to early December. Boso's signature coffee-coloured alkaline 'black water' fills the indoor and open-air baths; day use is a flat ¥700 for ages 3 and up, 11:00–15:00. Eat lunch in the dining room and the bath is free — spring bamboo-shoot dishes are the speciality. Five minutes by bus from Yoro-Keikoku Station on the Kominato Railway to Yorokan-mae. Lunchtime opening can be suspended at short notice, so phone ahead.
A hot-spring hotel on the cliffs of Inubōsaki, the easternmost point of the Kantō region. Perched on a cape jutting into the Pacific, its rock rotenburo and European-style open-air bath 'Opia' look out to the horizon and the Inubōsaki lighthouse; on New Year's Day you can watch Japan's earliest sunrise from the tub. The day-use course runs 12:00–19:00 (leave by 20:00), ¥1,500 for a 90-minute stay including bath towel, face towel and toiletries, so come empty-handed. Open daily (special pricing at Obon and New Year). Seven minutes on foot from Inubō Station on the Chōshi Electric Railway; combine with a seafood lunch in Chōshi and the 'Hill Where the Earth Looks Round' observatory.
A big spa resort on the cliffs of Cape Inubō, the first place on Honshū to see the New Year sunrise. The panoramic 'Ichibō-no-yu' main bath sweeps 180 degrees across the Pacific, and with the open-air bath, bedrock-bath rooms and sauna you can watch the sun climb out of the horizon or gaze at Inubōsaki lighthouse while soaking. The water is the sodium-chloride Byōbugaura spring. Adults ¥1,500 on weekdays, ¥1,700 on weekends, holidays and high season (cheaper for morning baths and after 17:00); open 10:00–23:00 (last entry 22:00), and on Saturdays you can stay through to 08:00 the next morning; open every day. Restaurants, banquet rooms, relaxation treatments and an ocean terrace make it a full-day stop to pair with the Chōshi Electric Railway, wet rice crackers and seafood. About 10 minutes on foot from Inubō Station; about an hour by car from Itako IC.
The Umi no Kairō bathhouse of this large resort hotel on the Pacific shore of Kamogawa, on the outer coast of the Bōsō Peninsula, welcomes day visitors. Fed by Kamogawa Onsen, tapped in 2003, its open-air bath faces the ocean with an uninterrupted view of the horizon and breaking waves; there are also an indoor bath, sauna and a glass-walled lounge looking out to sea. Adults (junior-high age and up) pay ¥1,650 and children aged 3–12 ¥825, rising to ¥1,760/¥880 in peak periods such as Golden Week, summer and New Year. Towels are not included – rent a set for ¥330 or bring your own. Normal hours are 11:00–22:00 (last entry 21:00), but on weekends, holidays and busy days bathing is split into 11:00–15:30 and 19:30–22:00. Open every day, with entry limits when crowded, and day plans with spa treatments or lunch are available. About 5 minutes by shuttle or car from Awa-Kamogawa Station on the JR Sotobō Line, close to Kamogawa Sea World – a fine finish to a day at the beach.