The Yagen Valley (薬研渓谷, Yagen-keikoku) is a remote mountain valley in the middle of the Shimokita Peninsula, on the Japanese island of Honshu.

There are only two tiny settlements in the entire valley: Yagen Onsen (薬研温泉), a tiny hot-spring hamlet, and Oku-Yagen (奥薬研), population zero but featuring some more hot springs. These hot springs have more than 400 years of tradition.

The local (Ohata town) tourist association has a Japanese-only guide site.

There are two paved roads into the valley. The main route from Ohata (15 km away) and Mutsu has one bus per day at 08:40, while the shortcut connecting to Mount Osore has none. A dirt trail, navigable by 4x4 in summer, traverses through the valley to the remote west coast.

Oku-Yagen and Yagen Onsen are 2 km apart and connected by a paved road and a forest trail. Most of the time it will be faster to walk than wait for the bus.

Oku-Yagen has 3 excellent open-air hot springs, all offering views of the Ohata River rushing through the valley:

• Meoto Kappa-no-yu (夫婦かっぱの湯), near the end of the paved road, is a modern town-operated facility offering sex-segregated open-air baths with dressing rooms and showers. Open 08:30 to 18:00 daily except Tuesday. Entrance fee is ¥230 for adults.
• Ganso Kappa-no-yu (元祖かっぱの湯) is the original hot spring, past the bridge at a fork of the river. The baths consists of two bare-bones dressing cabins and a large rock pool of hot water. This formerly mixed spring bath has adopted a sex-segregated model, where certain genders bathe at certain hours of the day. Attendants from Meoto kappa no yu will periodically check to ensure the rules are complied with, and a closing time, while posted, may or may not be enforced.
• The unnamed third hot spring is on the river side of the road before the other two, unmarked except for a break in the guardrail and stairs descending towards the river. While it once consisted of two concrete tubs formed into the surrounding rocks, the concrete has since deteriorated and washed away in the years following the Tohoku earthquake. The springbox itself is intact, however, and the water flows freely down a few natural miniature ponds worn into the rock. The industrious visitor can dam up the gap in the rock where the water exits into the river with rocks and debris and a pool will soon form deep enough to sit in. The "pool" is still clearly visible from the trail on the opposite river side, so the modest may wish to wear some appropriate swimming clothes, or visit at night.

Pretty much the only other form of entertainment is hiking. A very picturesque forest trail runs along the side of the river between the two hamlets, partly on the tracks of an old mining railway. The fall colors in the valley are particularly famous.

The local speciality is wild boar (inoshishi). There are only 3 places to eat, drink or shop in the entire valley:

• The Meoto baths have a simple noodle shop selling, well, noodles... and, perhaps more importantly, cold beer.
• The souvenir shop in Yagen Onsen sells souvenirs, drinks and more noodles.
• The liquor store in Yagen Onsen, open intermittently, sells booze.

Most visitors eat and drink at their own lodgings. If you're camping, bring your own supplies.

• Yagen Campground (薬研野営所, Yagen-yaeijo), at the entrance to the valley within walking distance of the springs, offers lawn space for ¥500 per person, toilets and little else. An excellent budget option if you're self-contained.

Yagen Onsen has several midrange minshuku/ryokan, offering a tatami for the night and two meals for ¥8000 plus. Many close for the winter season.

• Yagensō (薬研荘), ☏ +81 175-34-2779. Open April to November. ¥8400/4500 with/without meals. 

• Mount Osore, the gateway to hell of Japanese myth, is a half-hour ride away on a squiggly mountain road.
• Mutsu
• Sai
• Oma
• Kazamaura
• Higashidori
• Yokohama
• Rokkasho
• Noheji