They have a term for this in Japan. It's called Gaijin.

Don't worry. If you hear Dame! Dame! you might want to go someplace else......

I've got some minor tattoos, and although my mom makes a big deal about covering them up (my mother being from Japan and overly concerned about such things), I've never caught anyone looking. If you've got pretty elaborate tattoos, I'd imagine you'd get some looks here and there, but I wouldn't worry too much about it. Their pop stars are starting to get into the trend, so the tattoo's meaning is finally diversifying. :)

As for recommendations, I don't know about Tokyo, but there are quite a few towns you can travel to that focus on the onsen. My favorite, if you're traveling near mid-Japan, is a town called Gero. It's simply beautiful.

Cheers,
MariNaomi

I know a number of people who have been turned out of bathhouses for even small tattoos (think half-dollar size ankle piece).
Many bathhouses have signs on the door, saying "no tattoos" or "no ink."
However, small, local bathhouses, and onsen with private baths don't tend to mind. Just stay away from bright shiny new, mass-marketed looking places. Those tend to be the most strict and rules-heavy.

My best suggestion is go to a hotel that has private onsen rooms. It's expensive, but it's so great to have your own bath. And they'll even feed you in your own room, often enough.

From the sounds of things, your tattoo may cause a stir becasue of its similarity to Yakuza style tattoos. But yeah, smaller places are more likely to be flexible about tattoos.
If it were smaller, i'd say just cover it up, as one of my friends was asked to do when we were at an onsen. She has a lovely scarab with wings across her shoulders... they just wanted her to wear a towel over her shoulders so that the other patrons weren't bothered by it.

Anyway, even if they decide that your tattoos aren't ok in their bath, the worst they'll do is ask you to leave. They'll probably even be very polite about it.

I have a related, if more general, question about girls and onsen. My town has a (very) public onsen, at the edge of the river, alongside the main bridge. It's free and open to the public. Although it is quite common to see naked men hanging out in the bath, I have never, not once, seen a girl there. Well, maybe I saw a girl put her feet in once, but she was fully clothed. What's the deal? Are these sorts of onsen only for men?

Yeah, I saw Gero Onsen while doing a Tocoo search and was surprised! Doesn't "gero" mean "vomit"?

I wouldn't worry about it if you are going to onsens. There is a chance that they may turn you down if they see your tattoos when you try to enter the place, but once you're inside, chances are that they won't see you. Just be subtle and respect others, and no-one should give you a hard time. The fact that you're a foreigner will help too. They won't associate you with the Yakuza. I've never been hassled about my tattoo. And I'm Japanese. :)

Hope you have an awesome time - onsens are GREAT!! One of my favorite pastimes is hanging at the onsens for HOURS!!!

It is apparently worse at gyms and bathhouses - onsens don't seem to care as much..

My man and his mate went to spa world in Osaka and forgot about the no tattoo rule - my mate has huge tatts all over his legs...the girl asked and my mate said 'Oh I just have this little one on my knee'.

He rolled up his trouser leg and she went white and was shaking and saying 'ehhhh dame, dame desu yo, dekimasen'

needless to say they didn't get to enjoy spa world

I have to laugh, and have to post! I have no tattoos (though every year I swear I will get one for my birthday!). I do have a funny onsen story. While living in Tokyo I took a weekend getaway to Nikko and stayed at a marvelous ryokan - Konishi Ryokan, which is up on a hill directly above the Sacred Bridge. I fully prepared myself for the onsen experience, researching proper etiquette, etc. The first night I went down to the ryokan onsen in my yukata. I was the only one there! I dutifully sat on the stool and washed myself thoroughly, then entered the onsen. Shortly thereafter two women came in - a mother and daughter. They were very brave and used their english (so many in Nikko did - such welcoming people!). We had as much of a chat as is possible when there is only limited language and your boobs are bobbing on the surface of the onsen! It was lovely! The next day I was walking around the temples and these two women started waving at me from a distance - it kind of freaked me out. I finally realized it was the two women from the onsen the night before. I wanted to tell them I didn't recognize them with clothes on but I don't know how to say that in Japanese!

I have to agree with Leon. Be the one to make the change. It may take some guts, but its not going to make you too many enemies, and most people will respect you for it. The Japanese have a very distinct double standard when it comes to "gaijin." We (sic) aren't judged as strictly for our digressions because the Japanese themselves realize that their culture is so complex and esoteric that they don't expect foreigners to be very good at it to begin with. In fact, showing any kind of fluency in their customs or language will elicit an eruption of surprised praise.

Just go where you want to go, as long as your tattoos aren't visible when you pay to enter (on your arms or anything like that) you're home free. It would be extremely rude for anyone to ask you to leave the onsen once you are inside.