Before you start

A valid passport (the physical document — photos and scans are often refused for SIM registration)
An unlocked, eSIM-capable or dual-SIM phone helps if you want to keep your home number too
For a contract only: a NIE (foreigner ID number) and a Spanish bank account with an IBAN
For a contract only: usually a Spanish billing address

Step-by-step

1
Buy a prepaid (prepago) SIM with your passport on arrival
Walk into any Movistar, Vodafone, Orange, Yoigo or Digi shop (or a phone kiosk) and ask for a tarjeta prepago. The clerk registers the SIM to your passport on the spot — this ID registration is required by law, not optional. No NIE, no bank account, no contract. Airport stands work but cost more; a high-street shop is cheaper. Top up (recarga) at shops, supermarkets, ATMs or the carrier app.
In personWho: Anyone with a passport15-30 minutes, active immediately or within a few hoursFrom about €5-15 including starter credit
2
Get your NIE and open a Spanish bank account first if you want a contract
A monthly contract is billed by direct debit (domiciliación bancaria), so the carrier's system needs a NIE and a Spanish IBAN — applications without them are auto-rejected. Sort your NIE and open an account (a Spanish bank, or a fintech like N26/Revolut with a Spanish IBAN) before you try to sign up. Some operators also ask for proof of a Spanish address.
In personWho: Newcomers planning a long-term contractDepends on NIE and bank timelines — typically a few days to a few weeksNo telecom cost at this stage
3
Sign up for a contract (contrato) — Digi is the value pick
Once you have a NIE and IBAN, take out a contract online or in a shop. Digi is the carrier newcomers gravitate to: very cheap, no permanencia (no minimum commitment) and easy fibre+mobile bundles. Roughly ~€5 for 25GB or ~€7 for 50GB mobile-only. Movistar, Vodafone and Orange cost more but have the strongest networks; Yoigo, Lowi, Pepephone and Simyo are other cheaper options.
OnlineWho: Residents with a NIE + Spanish IBANSame day to a couple of daysRoughly €5-30 per month depending on data and operator
4
Keep your number: port it (portabilidad) when you switch
You don't have to lose your prepaid number when moving to a contract, or when changing carriers. Ask the new operator for portabilidad and give them your current number and SIM details — they handle the switch, usually overnight, and the change is free.
OnlineWho: Anyone changing carrier or planTypically 1 business day (often overnight)Free

Documents you’ll need

Passport — physical original, required for any SIM registration (prepaid or contract)
NIE / TIE (foreigner ID number) — needed for a contract
Spanish bank account IBAN — needed for a contract's direct debit
Proof of Spanish address — sometimes requested for a contract

Things most newcomers don’t know

The honest newcomer playbook is prepaid-now, contract-later: passport SIM on day one, then port to a cheap Digi contract once your NIE and IBAN exist.

It dodges the chicken-and-egg trap — a contract needs a NIE and Spanish bank account you won't have yet, while a prepaid SIM needs neither, so you're connected immediately and lose nothing by switching later.

Source: Go! Go! España / expat guides

Digi is the carrier newcomers and budget-conscious locals flock to — rock-bottom prices, no minimum commitment.

Digi is aggressively expanding in Spain, laying its own fibre; plans like ~€5 for 25GB with no permanencia can save €100-200/year versus the big three for comparable service.

Source: digimobil.es; Spain Expat

Registering a prepaid SIM with a foreign ID card has gotten hard — bring your passport, not just an EU/foreign ID card.

Operators tightened acceptance under the ID-registration rules; Vodafone and Movistar in particular often insist on a passport, while Orange and others are more lenient.

Source: Prepaid Data SIM Card Wiki (Spain)

Your Spanish SIM 'roams like home' across the whole EU/EEA — same domestic price, no extra charge.

EU 'Roam Like At Home' rules cover all EU states plus Iceland, Liechtenstein and Norway, so a Madrid plan works on trips across Europe — subject only to a fair-use data cap on very cheap/unlimited plans.

Source: European Commission — Roaming policy

Common mistakes to avoid

Trying to sign a contract before you have a NIE and Spanish IBAN — the carrier's automated system simply rejects the application.
Showing up to register a SIM with only a photo or photocopy of your passport — the original physical document is required, and foreign ID cards are increasingly refused.
Assuming eSIM is available everywhere — major carriers generally support it, but availability and in-store-only activation vary by operator and plan; carrier eSIMs still require the same ID registration.
Buying at the airport and overpaying — airport SIM stands are convenient but pricier; a high-street shop or supermarket gives better value and the same passport-only process.