Setting a passkey for your Nintendo account is pretty simple and straightforward, though it requires an iPhone running iOS 16 or better, an iPad running iPadOS 16 or better, a Mac running macOS 13 or better, or an Android device running Android OS 9 or better. On the device that you want to have the passkey registered to, go to Accounts.Nintendo.com and sign in. From there, go to Sign in and security settings > Passkeys > Register a new passkey. Enter your password at the prompt, choose “Register” after reading the disclaimer, and then follow the onscreen instructions.
Nintendo allows a maximum of 10 passkeys per account. If you’re told that you can’t add a new one, it’s because you’ve hit that limit. The “Passkey has already been registered” message, meanwhile, means that you’ve already registered a passkey on that device.
For additional account security, it’s a good idea to turn on two-factor authentication, as well, although that option is not available on what Nintendo deems “child accounts” (for ages 12 and under). To do this, go to Accounts.Nintendo.com, and, once signed in, choose Sign-in and then select security settings > 2-Step Verification > Edit > 2-Step Verification setup. From there, you need to have an authenticator app on your phone to use with Nintendo’s two-factor authentication process. (Though Nintendo specifically names Google Authenticator, others, like Microsoft Authenticator and 2FAS are at least as good and realistically, are probably better options with more features.) Scan the QR code with your authenticator app, follow the onscreen instructions, and you’re good to go.