The short answer
Pack for the season you are actually landing in, because Japan has four sharp ones, and bring shoes you can slip off twenty times a day. August in Tokyo runs above 31°C and humid, the tsuyu rains fall from early June to mid-July, typhoons come from June to December, peaking from July to September, and Hokkaido winters are long with heavy snow. Add a plug adapter for the flat two-pin sockets on 100 V, a small bag for your own rubbish because public bins are rare, and cash for the places that still want notes. Then check your medication: Japan bans some everyday cold and allergy medicines, and that is the one packing mistake with real consequences.
Clothes for four seasons and the shoes-off rule
Japan has four distinct seasons, so the list changes completely with the month you fly. Most of Honshu, Shikoku and Kyushu is humid subtropical: light breathable layers for a sticky August above 31°C, a rain shell for the tsuyu from early June to mid-July and for typhoon season, which runs from June to December, and a real coat for a dry, clear Pacific-side winter. Hokkaido is colder again, with long winters and heavy snow. You will walk much further than you expect, so bring comfortable shoes that come off easily: you take them off at homes, ryokan, temples and some restaurants, which also means socks without holes.
Power, and the small things that make Japan easier
Japan uses Type A and Type B plugs, the flat two-pin style, at 100 V, with 50 Hz in the east around Tokyo and 60 Hz in the west around Osaka. European devices need an adapter. Phone and laptop chargers handle 100 V without complaint, but anything not rated for it, hair dryers and straighteners above all, will underperform, so leave them and use the hotel's. Bring a power bank, because you will lean on your phone for maps, translation and train times all day. Two small items earn their space: a bag for your own rubbish, since public bins are rare and you carry it until you find one, and a compact towel for onsen and washrooms.
Medication: check it before you fly
Some common prescription and over-the-counter medicines are illegal to bring into Japan under a strictly enforced law on anti-stimulant drugs. The UK FCDO names Vicks inhalers, medicines for allergies and sinus problems, cold and flu medication containing pseudoephedrine, and some over-the-counter painkillers containing codeine. Foreign nationals have been detained and deported over this, so it is not a formality. If you travel with medication, check its status with the Japanese embassy in your country before you fly, carry everything in its original packaging with a copy of your prescription, and do not assume a Japanese pharmacy will sell you an equivalent.
Your pre-trip digital checklist
Do the digital admin at home on your own wifi, not in the arrivals hall. Visit Japan Web, run by the Digital Agency, files your immigration and customs declarations in advance and gives you a QR code. Download offline maps of the cities you will visit, install your eSIM so you land already connected, and save your accommodation and flight details where you can open them without data. Pack a passport holder you will actually use: you must carry your passport at all times, and police can arrest you if you cannot show your status. Leave bulky clothes at home, laundry is easy, and leave the tipping habit behind too.
