Is it rude to give up your seat in Japan?

First time I've heard of this, and I think it's nonsense. There is a strong social convention that people should give up their seats (not just the designated priority seats) for elders, very young kids, the disabled/injured and pregnant. Nobody will be offended or think you rude for doing that. They might call you out if you don't. The recipient most likely will say "sumimasen" which literally implies indebtedness, but that's just a set phrase.

Of course, you are effectively declaring the recipient infirm, which they might take objection to - there's a big gray area where age is concerned, which the Japanese themselves are unsure about and sometimes debate. In fact, even the very elderly will often reply "I'm not that old" and reject an offered seat at first, but the expectation is for you to insist that they take it - quite similar to what happens during gift-giving. In some cases, people might feel uncomfortable about not being able to go through this routine when offered a seat by a foreigner (who's not expected to speak Japanese).

The only case where offering a seat might really be considered rude (or at least awkward) is when the recipient clearly is young and healthy. Why are you doing it then? If it's a woman, are you making a pass at her? Even then, should be OK if you're in a similar age range and doing it in a charming manner, not so if you're much older or being creepy.

I live in Japan (Tokyo) and no one gives up their bus or train seats unless the standing person is clearly incapable of standing for long (old, injured, pregnant). Then they are fairly good about it. What's really entertaining is watching two elderly people with canes / walkers etc. arguing about which one of them need the seat more. And it's the "good" argument: "No, you should take the seat." -> "I'm fine, I can stand until Daitobunkadaigaku. You should take the seat!" And on it goes.

I give my seat pretty often in Tokyo.

I can tell that it is never rude.

On the contrary they are very grateful, so much that usually I prefer to keep a distance afterwards.

Also sometimes they won't accept it to avoid bothering you, so I insists and say that I will get off soon anyway.

No, I do it regularly. There are even spaces on trains and buses that are marked for the elderly, pregnant, disabled, etc. where the able-bodied may sit but are to give up their seat if anyone in greater need of it shows up.

It is not uncommon that the person you are giving the seat up to may initially refuse your kindness because it is polite to be slow to accept certain forms of assistance -- and occasionally you might want to give your seat up to someone who really doesn't feel like sitting (they may have already been sitting all day) -- but it is never rude to offer, unless the way in which you offer it is particularly nasty.

I suppose this idea could have come from the custom of sharing hardship. For example, if you meet a friend walking in the rain and he has an umbrella and you don't, he may close his to share the rain with you instead of being the only one dry. Its the same with eating in front of someone who has no food -- its considered rude. But that's not at all the same thing as giving your seat up to a stranger.

Nowadays, overtly “giving up one’s seat” risks abuse almost everywhere^†. So, don’t do so, and not just in Japan. Instead just get up and walk away, if you can, otherwise just stand up. There is no need for “really this is my seat but I am prepared to let you have it”.

^† Example here for Pune.

That is most likely true in most situations. However, the general idea is to give up your seat to someone else that may need it more than you such has pregnant women and older people. Also, there are certain sections in the train that are reserved for those people. If the train is too crowded then you may face a situation where those people cannot get to those seating areas, then you should definitely offer your seat at least. Give them the option to decline.

Lots of good thoughts here. To which, I would add, anyone who seems to need it more than you do. I've given up seats to people holding infants, pregnant women, the elderly, people with canes or braces / casts on body parts, people traveling with small children, and so on. And I always consider myself the 'winner' in the transaction since it just feels good.

And I happily accept any similar offer made to me!