Can a US citizen work remotely for a US company while in Japan for sightseeing?

The word of the law is quite clear: a person on Temporary Visitor status cannot engage in

activities related to the management of a business involving income or activities for which he/she receives remuneration.

None of these terms is given a special definition in the text, so we may assume that they are to be understood in their ordinary meaning, and the answer to the question in the title is no.

However, I am not aware of any case where someone in that kind of situation was charged, so it is unknown what a court would say.

Please see the Japanese Embassy linked below to answer your question- yes, you are allowed to work for 90 days in Japan with a tourist visa.

http://www.us.emb-japan.go.jp/english/html/travel_and_visa/travel_and_visa_index.htm

Yes- my [non-lawyer, non-legal, non-everything else] estimation is it's akin to being salaried but spending the 90 days allowed from a tourist visa in the country. As long as you aren't changing your business practices in any way due to being in Japan or somehow engaging in business there, you are essentially maintaining your employment. However, if you cycle through tourist visas by exiting and reentering the country after 89 days repeatedly, you could be in trouble. At that point you would be enjoying the spoils of Japanese taxpayers without contributing.

However, no one on this site can give you legal advice. If you want to be 100% sure, get a free answer from a Japanese visa company here-- http://www.juridique.jp/formeng.html

even better, consult the official Japanese Embassy in the US' website. It states clearly that staying in the country for 90 days- even for business explicitly- is perfectly fine.

"A visa is NOT necessary for US passport holders visiting Japan for a short-term stay of less than 90 days with the purpose of tourism and business."

http://www.us.emb-japan.go.jp/english/html/travel_and_visa/travel_and_visa_index.htm

De jure there are dozens of complex laws and regulations regarding remote employment, under which you may or may not need a special visa in order to work in a given country. Tax laws are an additional complication, where countries such as the UK can deem you as a tax resident for spending as little as 16 days on British soil.

De facto, as long as you don't mention your remote job to immigration personnel at the airport, there's a 99.99% chance no one will ever find out. There are millions of people breaking the law by being employed at on-site jobs in any given country, so digital nomads are a pretty low priority for law enforcement.

No, it's not allowed. But what is considered work? Replying to and reading emails? Or surfing the internet and reading up new stuff related to your job? Where is the boundry when it turns from work for free, which is allowed, to work for money which isn't: when you receive money or when you're actually working on something? How can the police find out how much time you've been doing your work and how much time you've been learning new stuff? I advice you not to think about that and don't mention it to the police or in the airport in Japan, and anywhere else. For them you're a tourist and your purpose of visit is tourism. And keep doing your job.