So...without further ado - here's my little guide to using your ATM card in Japan!
Notify your financial institution that you're heading overseas!
This is a big one - I worked in the Card Fraud department at a bank for several years, and if we noticed transactions coming through from international locations we would often put a temporary stop on a cardholders account until we could verify with them that the transactions were legitimate. The last thing you want when you're trying to use your card overseas is it declining because your bank doesn't know that you're actually in another country! All it takes to prevent that is popping in to your local bank branch or calling their contact center - the staff member there will generally just put a note on your account that you're going to be overseas during a certain timeframe. Problem solved!
ATM acceptability for international issued cards can be hit or miss
I have both an Australian issued bank card and a US issued bank card and I've found that 7-11 and Japan Post ATM's are both reliable spots to use my foreign bank issued cards. I've had occasions where I have tried to use them at other Japanese Bank ATMs and they have been straight up declined - so my best advice to you international travelers would be to go straight for a 7-11 or Japan Post ATM to get your cash out. Those ATMs also make life easy because they'll give you an option as soon as you put your card in to have the menu prompts in English. Keep in mind that Japan Posts are usually open within standard business hours, so if you're trying to head to a JP location outside those times you might find the ATM is inside and unable to be accessed. 7-11's being 24/7 are often a bit more reliable in that sense!
Also double check with your bank about the fees they charge for international ATM withdrawals
My Australian bank for instance charges me a flat $5 ATM withdrawal fee if I'm taking cash out. As you can imagine if you're taking out small amounts here or there, that's going to add up fast. You want to find a happy medium somewhere between not having a ton of cash to carry around, but not using the ATM excessively so that your balance is being eaten up by fees. If you're lucky enough to have a bank that doesn't charge you an international ATM withdrawal fee then that's even better!
Remember that a lot of places in Japan are still cash-only establishments
This was one thing that took me by surprise initially when moving here. A lot of the big retailers will take card, but there have been places I would have thought would definitely take credit cards but they didn't. You will need to have cash on you to avoid getting into a situation where you don't have a means to pay. Usually if a place does take card, you'll see it on their door or by the entrance with the typical logos for Visa/Mastercard and so forth. If you do want to ask the staff member at a store or restaurant if they do accept cards (particularly handy if your cash stash is getting low and you're wanting to try and save it if you don't have to use it!) you can ask "Kurejitto kaado wa tsukae masu ka?" which translates to "do you accept credit cards?"
Hopefully these tips save you some dough and some stress when it comes to using your card in Japan!