Before you start

A Spanish NIE/TIE and, normally, a Spanish employment contract and Social Security registration
Spanish tax residency (generally triggered by spending more than 183 days in Spain in the calendar year)
For the Beckham Law: you must not have been a Spanish tax resident in the previous 5 tax years and must move for work reasons
A digital certificate, Cl@ve, or DNI/NIE electronic ID to file online with the AEAT

Step-by-step

1
Confirm whether you are a tax resident
You are a Spanish tax resident if you spend more than 183 days in Spain in a calendar year (days need not be consecutive) or your main economic interests are here. Residents pay IRPF on worldwide income; non-residents pay only on Spanish-source income. This single test drives everything else.
OnlineWho: You (self-assessment; confirm with a gestor if your move date is borderline)Assess for the calendar year of your arrival
2
Let your employer set up payroll withholding (retención)
Spain runs a strict pay-as-you-go system. Your employer estimates your annual tax and deducts IRPF from each paycheck, paying it to the AEAT. You give them Modelo 145 (personal/family data) so the rate is correct. Most employees do nothing else during the year.
Via employerWho: Your employer's payroll/HR; you supply Modelo 145From your first payrollWithheld from salary (ranging roughly 2% to 45% by income)
3
Decide on and elect the Beckham Law within 6 months
If you qualify, file Modelo 149 to opt into the special regime for displaced workers within 6 months of registering with Spanish Social Security. You then pay a flat 24% on Spanish employment income up to €600,000, and 47% on the excess, for the arrival year plus 5 more (6 years total). The deadline is strict — a late filing disqualifies you entirely.
OnlineWho: You (most people use a tax adviser/gestor)Within 6 months of Social Security registration / start of workForm is free; advisers typically charge a few hundred euros
4
File your annual income tax return
Standard residents file the annual IRPF return, Modelo 100 (the Renta), reporting worldwide income and reconciling against what was withheld, between early April and 30 June for the prior year. If you elected the Beckham regime you instead file Modelo 151, which reports only Spanish-source income at the flat rate.
OnlineWho: You, via the AEAT Renta WEB portal or a gestorApprox. 2 April to 30 JuneFiling is free; you pay or are refunded the balance vs. withholdings
5
Handle home-country obligations and information returns
Spanish tax residency does not erase home-country duties. US citizens and green-card holders must still file a US return every year (using the FEIE and/or Foreign Tax Credit) plus FBAR if foreign accounts exceed the threshold. Spanish residents with significant assets abroad may also owe the informational Modelo 720.
OnlineWho: You (a cross-border accountant is strongly advised for US persons)Annually, alongside the Spanish return

Documents you’ll need

NIE/TIE and passport
Spanish employment contract and Social Security registration (alta) number
Modelo 145 (given to your employer to set the correct withholding)
Modelo 149 to elect the Beckham Law, then Modelo 100 (standard) or Modelo 151 (Beckham) for the annual return

Things most newcomers don’t know

Madrid is one of Spain's most tax-efficient regions for high earners even without Beckham.

Half of your IRPF is set by the autonomous community, and Madrid sets the lowest regional rates — a combined top marginal rate of about 45%, versus 47%+ elsewhere. Where in Spain you register genuinely matters.

Source: AEAT IRPF manual (Comunidad de Madrid) / TaxDown

The Beckham Law is a large saver but is opt-in and time-boxed: miss the 6-month window and you're stuck on progressive rates for the whole stay.

It replaces a top rate near 45-47% with a flat 24% up to €600,000. Because it must be elected via Modelo 149 within 6 months of Social Security registration and the deadline is unforgiving, it is the single most valuable thing to sort out on arrival.

Source: Immigration Spain / Greenback

Under Beckham you generally are NOT taxed on most non-Spanish income, and you file Modelo 151, not Modelo 100.

The regime treats you broadly like a non-resident for foreign income, which is why it suits people with overseas investments — but it also means a different annual form.

Source: Immigration Spain / PwC

For US citizens, paying LESS tax in Spain under Beckham can paradoxically raise your US bill.

The US taxes worldwide income for citizens regardless of residence. The lower your Spanish tax, the fewer foreign tax credits you generate to offset US tax, so the 24% flat rate can leave a residual US liability. Coordinate FEIE/FTC with a cross-border accountant.

Source: Greenback Tax Services

Common mistakes to avoid

Missing the 6-month Beckham deadline (counted from Social Security registration or start of work) permanently forfeits the regime — there is no second chance for that stay.
Assuming withholding equals your final tax: retenciones are only advance payments; you must still file Modelo 100 (or 151) and may owe more or be due a refund.
Forgetting that crossing 183 days makes you taxable on worldwide income, not just Spanish salary — which can sweep in foreign rental, investment and capital-gains income.
Autónomos (self-employed) cannot use the standard employment Beckham route and face different rules, quarterly IRPF payments (Modelo 130) and VAT.