Kirk here with a Kumanichi article about tattoos and onsen. Main takeaways:

  1. Kumamoto tends to be conservative, with few onsen allowing people with tattoos;

  2. There’s an onsen in Kumamoto (unnamed in the article) that allows people with tattoos to bathe if (a) they are not Japanese and (b) they are able to hide their tattoos with seals.

As an effort to accommodate folks with tattoos the seal idea might be better than nothing but, even though I don’t happen to have tattoos myself, I would think it would be pretty much take the pleasure out of a bath if one had to sheepishly hide one’s tattoos (culturally identifying symbols in some cases) with plastic seals.

https://kumanichi.com/articles/895948

P.S. If you are interested in finding places that allow tattoos you can find some tips by searching this page with the word “tattoo”:

https://www.facebook.com/page/123734781033296/search/?q=tattoo