US to Japan to China to South Korea

Yes, you can, but your immediate destination after Shanghai has to be in a different country or eligible territory than Japan. In other words, your itinerary, starting from Japan has to have three different countries in a row:

A (Japan) - Shanghai (China) - X city (country different from both Japan and China and it can be Taiwan, Hong Kong and Macau as well)

It depends on where you are going after Shanghai. If you are going straight back to Japan then you need a visa. If you are going to another country (or Hong Kong, Taiwan or Macao) then you should be eligible for visa-free transit, assuming your nationality is also eligible.

Thanks so much for your help.
Our trip goes US to Japan to Korea to Shanghai to Japan to US.
Based on your answers, looks like we should be able to use 144 exemption rule. right?

Yes, you are eligible for the 144-hour visa-free transit.

I have the exact same itinerary but my tickets are NOT connecting flights. I got a one-way from US > Japan > Guangzhou. And then ANOTHER one-way ticket Guangzhou > South Korea > US. Will I have a problem since the flights are not connecting (like Japan > Guangzhou > South Korea). Or is it okay to have tickets like that?

Hi Vanessa, it is okay to have separate tickets.