The iPhone now packs Felica (a different beast from NFC), so paying with Suica straight from your phone has become a thing. Since the Suica lives inside the iPhone itself, you don't have to worry about forgetting or losing the card, and topping up from a credit card or Apple Pay is super quick and convenient — but what if you don't have a credit card? I dug around to find out which non-credit-card methods actually work.
This article is meant for anyone with an iPhone 7 or later (the ones with Felica), or an Apple Watch Series 2 or later.
1. Top up at a station ticket machine

This is the easiest option, but most ticket machines these days only take cards, so unfortunately not every station will let you top up.
Of course, it depends on the model — if the machine has the type of slot where you place your card down to top up, you're good to go.

Even outside JR East's Suica area, on networks like JR West, I was able to top up using this style of machine.

2. Top up at a Seven Bank ATM

Seven Bank ATMs actually have a built-in feature for topping up electronic money like Suica, and you can pay in with cash.
Tap "Electronic money charge" on the screen, then unlock your iPhone with Face ID or your passcode and place it on the reader. (On the iPhone X, it didn't recognize the device unless I held it upside-down.)
After that, just pick the amount you want to top up and feed bills into the ATM like you normally would.
Seven Bank ATMs are often installed inside stations too, so you don't have to go out of your way to find one — that's a nice bonus.
3. Top up at a convenience store

You can top up Suica at almost any convenience store across Japan.
Just hand it over at the register and ask them to top up your Suica.
It's a bit of a hassle if you'd rather not deal with a clerk, so it depends on where you are.
4. Top up with a debit card
If you don't have a credit card, a debit card will also work for top-ups.
Once you register one in the Mobile Suica app, you can top up anytime — same flow as with a credit card.

I tried it with a Rakuten Debit Card (JCB) and it went through with no problems.
Debit cards don't require a credit check, and at most banks even high schoolers can get one issued, so if topping up with cash feels like too much hassle, getting one might be worth a shot.