Chubu Niigata
Sauna and Tennen Onsen Jonnobi-kan
サウナと天然温泉 じょんのび館
Name in Japanese. Look for this on the sign, or show it to a taxi driver.
Tattoos allowedStated by the facility
Super sento (large day-bath complex). Water source not checked; see the official page

Tattoo rule
Tattoos allowedStated by the facility
What the official page says
当館は新潟市が設置する公の施設であることから、ご入館に際して刺青・タトゥー のある方も特段の制限を設けておりません。
In short: As a public facility set up by Niigata City, it places no particular restriction on guests with tattoos.
Where it is
Chubu region, Niigata prefecture. About 188 km in a straight line from Sendai.
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Rules can change. Check the official page on the day you go. Any paid link is labelled next to the button. We have not visited this bath ourselves.