In Japan: get an eSIM before you go.
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An eSIM is the better choice for Japan. Japan has excellent eSIM coverage via Airalo and Ubigi, and physical tourist SIMs are also easy to buy at the airport with no registration hassle.
No — Japan doesn't require passport/ID registration to get connected.
Buy your eSIM online before you fly (Airalo and similar marketplaces cover Japan) and install the profile while you still have WiFi — it activates the moment you land.
Only if you're fine paying your home carrier's international roaming rates — check your plan first, since those can add up fast compared to a local SIM or eSIM.
Airport SIM booths and roaming plans are the expensive way to get online. A Saily eSIM (by the team behind NordVPN) installs on your phone before you fly, activates automatically when you land in Japan, and gives you prepaid data at local prices in 190+ countries — no physical SIM, no contract, no surprise bill.
See Saily data plans for Japan →Disclosure: if you buy through this link, TravelSmarter may earn a commission at no extra cost to you. It keeps our tools free.