The essentials
IC transport card (Suica/Pasmo)
Load one up at any train station. Essential for getting to every stadium in Japan. Also works at convenience stores and vending machines inside stadiums.
Cash
While cashless payment is growing, many stadium food vendors still prefer cash. Carry at least ¥5,000 in coins and small bills for food and drinks.
Portable battery
You'll be using your phone for maps, translation, photos, and live stats. A portable battery is essential — Japanese games last 2-3 hours and you'll drain your phone fast.
Weather preparation
Rain poncho (outdoor stadiums)
Umbrellas block views and are frowned upon. Buy a clear rain poncho at a 100-yen shop (Daiso, Seria, Can Do) — they're sold near most major stadiums too. Essential for soccer and outdoor baseball.
Sun protection (summer)
Japanese summers are brutal — 35°C+ with high humidity. Bring sunscreen, a hat, a cooling towel, and a handheld fan. Many fans bring frozen water bottles that double as ice packs.
Warm layers (winter soccer/basketball)
Winter J-League and B.League games can be cold, especially in outdoor stadiums. Heat pads (kairo) are sold at every convenience store — stick them on your back under your jacket.
Stadium-specific items
Team towel
The muffler towel is Japan's universal fan accessory. Buy one at the stadium shop — it's the best souvenir and shows support. Costs about ¥1,500-2,000.
Clear bag
Some stadiums have bag size restrictions. A small clear bag or compact backpack under 30cm is safest. Check the specific stadium's rules online before going.
Binoculars
Compact binoculars are great for outfield seats at baseball games or upper-tier seats at large stadiums like Nissan Stadium (72,000 capacity).
What NOT to bring
Glass bottles are banned at all stadiums. Cans may need to be transferred to paper cups. Large flags or banners need advance permission. Selfie sticks are discouraged in crowded sections.
Summary
IC card, cash, portable battery, weather gear, and a team towel — that's your Japan sports survival kit. Everything else is available at convenience stores near the stadium.