Passport
NIE
In Spain, you or your gestor can actually set up payment of your taxes and social security directly from your bank. This is why you need a Spanish bank account; everything is all wired together.
I know what you're thinking, I'll set up N26 or Revolut and done! However, at least so far, none of these non-Spanish banks have the necessary agreements with the Spanish tax/social security agency to automatically deduct payments.
For me, getting a Spanish bank to give me an account was a nightmare, despite the fact that I hear from others all the time that it was easy for them. As with each step in this guide, you may need a quiet place to meditate when you're done, to prevent insanity.
I walked into many banks, BBVA, Caixa, Sabadell.. and different branches thereof. I even went with a local friend to help me translate my intentions. It's seems that you to need to be a resident to set up a bank account, but you'll normally need an account before you can achieve residency 🤷♂️
This contradiction is most easily solved by setting up a non-resident account, which many banks offer, and then transfering it to a resident account after you get residency. However, none of the banks seemed incentivised to give one to me, and I had a hard time convicing them. In the end, my landlord gave me an introduction to his bank manager at Sabadell, and they said they would give me an account on the condition that I took out a year worth of health insurance through them. It's as exasperating as it sounds. Spain is very much a country where it's easier to do simple things when you know people. Another member of the community had sucess with BankInter, who charged him 10€/month for the account.
04/11/22: I managed to set up a commission-free non-resident account at the Santander near the Corte Inglés. Needed to bring 1) passport, 2) Spanish address -- no empadronamiento required, 3) proof of residence in home country -- electric bill, credit card statement, or something else addressed to you, and 4) proof of income. Commission-free if you're under 30.
Setting up the resident account after obtaining the green NIE: When my first quarter of tax came due, my accountant said it could be automatically debited from my bank account, but it failed, causing a delay that cost me about €50. The problem was that my bank account was originally set up with my passport number. When I went to the bank to rectify this, they refused to associate the account with my NIE because I had not yet got the Green NIE (Certificado de registro de ciudadano de la Unión). After I got it, I went straight to the bank and converted the account to a resident account. It's important to remember to do this as soon as your Green NIE comes through.
Update 24/09/23
Some local bank companys provide account for non-residents.
Santander
https://www.bancosantander.es/en/particulares/cuentas-tarjetas/cuentas-corrientes/cuenta-no-residentes
A contract is only possible at santander office.
Passport and your own TAX number of your home country is required (Important for EU citizin).
Account can changed to a normal one after getting the "Residencia".
Caixa bank aka Hole Bank
HolaBank is the online "product" from caixa bank and they have accounts for non-residents.
https://www.caixabank.es/particular/holabank/online-banking.html
Online contractable.
Passport and your own TAX number of your home country is required (Important for EU citizin).
Account can changed to a normal one after getting the "Residencia".