The short answer: when a worker sits in Lisbon, Madrid, or Mexico City and logs in to a US employer's systems each morning, personal income tax almost always follows the worker's physical location — not the employer's. What varies is who withholds, whether the arrangement creates permanent establishment (PE) risk for the US company, which social security system collects contributions, and how double taxation is prevented. Those four questions have different answers depending on whether the person is on a US W-2 payroll, a 1099 contractor, or employed locally through an Employer of Record (EOR). This article walks through the mechanics as of 2026 for four common host countries — the United States, Portugal, Spain, and Mexico — and flags where a local tax adviser is not optional.
The core rule: work is taxed where it is performed
Under the OECD Model Convention, which underpins most US bilateral tax treaties, employment income is taxable in the country where the employee physically performs the work. The employer's country of incorporation is almost irrelevant to the employee's personal tax bill. A software engineer sitting at a desk in Porto is earning Portuguese-source income for personal-tax purposes, even if every dollar comes from a Delaware LLC and lands in a US bank account.
The country of tax residence adds a second layer. Most of the countries a US employee is likely to work from — including Portugal, Spain, and Mexico — tax residents on worldwide income. Residency is generally triggered by 183 days of physical presence, but every one of these jurisdictions has secondary tests (habitual abode, center of vital interests, availability of a home) that can pull someone in on fewer days. See how tax residency works for the mechanics.
The United States layers a third rule on top for one specific audience: US citizens and green-card holders remain subject to US federal income tax on worldwide income regardless of where they live. Non-citizens on a US payroll who move abroad and stop meeting the substantial-presence test (31 days in the current year plus a weighted 183 days across a three-year rolling window) generally exit the US personal-tax net for wages earned outside the US.
Employee vs contractor vs EOR: the structural fork
Before any country-specific analysis, the arrangement between the worker and the US company has to be classified. Three structures dominate.
W-2 employee working abroad on a US payroll
The simplest to describe and often the most problematic in practice. The US company keeps the person on the domestic payroll, withholds US federal income tax and FICA (Social Security and Medicare) as if the employee still lived in the US, and issues a W-2 at year end. The employee then files a foreign tax return in the host country and typically owes tax there as well, because that is where the work is physically performed.
Two problems arise. First, the US company is generally not registered as an employer in the host country and has no payroll infrastructure to withhold local income tax and social contributions — obligations that often exist regardless of the employer's own presence. Second, the US company creates PE risk (discussed below) by having a person perform substantive work on its behalf inside another country.
1099 independent contractor
The US company treats the worker as an outside vendor, pays gross with no withholding, and issues a Form 1099-NEC (if the contractor is a US person) or Form 1042-S / nothing (if the contractor is a non-US person performing services entirely outside the US). The worker then invoices the company from whatever business form the host country requires — a Portuguese trabalhador independente, a Spanish autónomo, a Mexican persona física con actividad empresarial.
Contractor status shifts self-employment tax, VAT registration, and social contributions to the worker. It reduces the employer's PE exposure meaningfully but does not eliminate it — most treaties look at economic substance, not paperwork labels, when deciding whether a dependent agent creates PE. Misclassification risk is real; several of the countries below have aggressive re-classification regimes.
Employer of Record (EOR)
A third-party company — Deel, Remote, Velocity Global, Rippling EOR, or a local equivalent — becomes the legal employer in the host country. It runs local payroll, remits local income-tax withholding and social contributions, issues a local employment contract, and invoices the US client company monthly for salary plus a markup (typically 10–20% or a flat per-employee fee). The worker is legally employed by the EOR under the host-country's labour law; the day-to-day economic relationship is with the US company.
EORs generally provide the cleanest tax outcome and the strongest defense against PE assertions, at the cost of a per-employee fee, reduced flexibility on equity compensation, and dependence on the EOR's own compliance quality. They are not a magic shield — tax authorities in several EU jurisdictions have begun challenging EOR arrangements where the substantive employer-employee relationship clearly sits with the client company.
Permanent establishment risk: what the US employer creates
PE is a treaty concept that gives the host country the right to tax the profits of a foreign company that has a sufficient economic footprint there. A single remote worker performing back-office tasks rarely creates PE on their own. Risk climbs when the worker: negotiates or signs contracts on the company's behalf, manages a local team, holds a customer-facing title (Country Manager, Head of Sales EMEA), or works from a fixed office that the company effectively controls.
The stakes are meaningful. A PE finding typically means the US employer must register with the local tax authority, allocate a portion of global profits to the local branch, file corporate returns, pay local corporate income tax (currently 19% in mainland Portugal, 25% in Spain, 30% in Mexico), and — depending on the country — become liable for VAT and payroll-tax registration.
The three commonly-cited defenses:
- Preparatory or auxiliary activities. Article 5(4) of most treaties carves out functions like data collection, market research, or purely administrative work. This is narrower than employers assume; anything commercially core to the business rarely qualifies.
- Independent contractor structure. A genuine 1099 relationship — with the contractor serving multiple clients, controlling their own methods, and bearing business risk — sits outside the dependent-agent PE rules. Sham independence, where the "contractor" works exclusively for one client on directed terms, does not.
- EOR employment. Because the person is legally employed by a third party, the argument goes, the US company has no employees in the country. Tax authorities in Spain and several other EU states have started challenging this when the client company exercises full operational control.
Social security and totalization
Personal income tax and social security follow separate rules. The US has bilateral totalization agreements with 30+ countries — including Spain and Portugal — that determine which country's social system a cross-border worker pays into and prevent double contributions. There is no totalization agreement between the US and Mexico as of 2026, which is a live issue for the many US employees working remotely from Mexican cities.
The typical treaty outcome: a US-employed worker sent abroad for up to five years on a temporary assignment can obtain a Certificate of Coverage and continue paying US FICA (Social Security and Medicare) while being exempt from the host-country system. Longer or open-ended assignments generally shift contributions to the host country. A local hire, EOR employee, or independent contractor is almost always in the host country's system from day one.
The financial stakes are not trivial. Employer-plus-employee social contributions run roughly 34–35% of gross salary in Portugal, close to 37% in Spain, and around 30% in Mexico when payroll taxes and IMSS contributions are combined — often exceeding the income-tax burden itself.
Country-by-country outcomes
United States: what happens to a US citizen abroad
A US citizen working for a US employer from another country stays inside the US tax net no matter what. Federal rates run 10–37% (the TCJA seven-bracket structure was made permanent by the One Big Beautiful Bill Act signed 4 July 2025), plus state tax if state residency has not been cleanly severed — see state residency severance for how to break California or New York before moving.
Two mechanisms prevent economic double taxation. The Foreign Earned Income Exclusion (FEIE) is $132,900 for the 2026 tax year and requires either the physical-presence test (330 full days abroad in any 12-month period) or bona fide residence status. The Foreign Tax Credit allows a dollar-for-dollar credit for foreign income tax paid, subject to sourcing and category limits. For high earners in a high-tax host country, FTC is usually the better choice; for lower earners in a low-tax country, FEIE often wins. The trade-offs are unpacked in the FEIE vs Foreign Tax Credit comparison.
Portugal
Personal income tax runs on a progressive scale up to 48%, with an additional solidarity surcharge of 2.5–5% on income above €80,000 and €250,000 respectively. Tax residency triggers at 183 days or by having a home available on 31 December. Portuguese residents are taxed on worldwide income.
The old NHR regime closed to new applicants from 1 January 2024. Its replacement, IFICI (sometimes marketed as "NHR 2.0"), offers a flat 20% rate on qualifying Portuguese-source employment and self-employment income for ten years, but eligibility is materially narrower — restricted to specific scientific, innovation, and higher-education activities, and closed to anyone who was Portuguese tax resident in any of the prior five years. See the IFICI explainer for who actually qualifies. A general remote worker on a Digital Nomad (D8) visa does not automatically get IFICI.
Spain
General progressive rates reach 47% at the state level and up to 54% in autonomous communities such as Catalonia and Valencia. Savings income (dividends, interest, capital gains) is taxed on a separate scale from 19% up to 30% above €300,000. The Beckham Law — Spain's special inbound regime — offers a flat 24% rate on Spanish-source employment income up to €600,000 (47% above) for the first six years, and does not tax foreign income, but it only covers employment income; autónomo (freelance) income is excluded outside of narrow highly-qualified professional categories. The Beckham Law guide walks through eligibility and the 2023 expansion.
Spain also runs a Digital Nomad Visa which, from 1 January 2026, requires proof of at least €2,849/month in income. Spanish tax authorities have been notably assertive in challenging EOR arrangements and in pursuing individual worker misclassification. A wealth tax and a temporary solidarity surcharge on net assets above €3 million (extended through 2026) add friction for higher-net-worth remote workers, though Madrid and Andalusia effectively neutralise them through 100% bonificaciones.
Mexico
Residents pay progressive rates up to 35% across 11 brackets and are taxed on worldwide income. Residency triggers at 183 days or if Mexico is the center of vital interests — which the SAT interprets broadly, including where the person's primary income source is located. Non-residents pay a flat 25% on gross Mexican-source income unless they elect progressive rates on net.
The RESICO simplified regime is often decisive for remote contractors. Self-employed individuals with annual revenue under MXN 3.5 million (roughly $195,000 as of 2026) pay effective rates of 1–2.5% on gross revenue — dramatically lower than progressive rates — but must issue electronic invoices (CFDI) for every payment received. This creates a strong tax pull toward the contractor structure for remote workers in Mexico earning below the RESICO cap. The lack of a US–Mexico totalization agreement means US-payroll workers face a real risk of paying into both social systems; local hire, EOR, or contractor structures typically avoid this by placing the worker exclusively in the Mexican system.
The double-taxation outcome by structure
Combining the mechanics above, the typical outcomes look like this.
| Structure | Host-country tax | US tax (non-citizen) | US tax (citizen) | PE risk |
|---|---|---|---|---|
| US W-2, resident abroad | Full local tax on wages | None once substantial-presence test broken | Full US tax, offset by FEIE or FTC | High |
| 1099 contractor | Local self-employment tax + VAT + social | None if services performed abroad | Full US tax + self-employment implications | Medium |
| EOR (local employment) | Full local tax, withheld at source | None | Full US tax, offset by FTC (usually preferable) | Low |
Double economic taxation — the same dollar taxed by two countries with no relief — is rare when a treaty exists and is used correctly. Every country in this article has a comprehensive tax treaty with the US, and the treaty mechanics guide covers how the tie-breaker rules resolve residency conflicts. The more common failure mode is procedural: missed filings, missed foreign-tax-credit elections, missed totalization certificates, or unregistered payroll obligations that surface years later.
Practical implications
For a US company hiring one remote worker abroad, the pattern that minimises risk is usually EOR employment, with a written policy limiting the worker to non-customer-facing, non-contract-signing activities and no fixed office in the host country. For a worker moving abroad who wants to keep a US W-2, the honest conversation with the employer is whether the company is willing to accept PE and payroll-registration risk — many are not, and offer a choice between EOR conversion, contractor conversion, or return to the US.
For contractors, the local business form matters enormously: a Portuguese trabalhador independente outside the IFICI regime pays full progressive rates; a Mexican RESICO taxpayer pays 1–2.5%. The choice of jurisdiction can shift the effective tax rate by 30 percentage points for the same underlying income. As of 2026, verifying eligibility and staying inside the applicable regime's caps and reporting requirements requires local advice; every regime described here has annual filing steps that a foreign accountant cannot handle remotely.
Where to go next
For jurisdiction-level detail, the country pages have current rates and regimes: United States, Portugal, Spain, and Mexico. The tax planning for remote workers guide covers residency mechanics in more depth. For side-by-side comparison, use the country compare tool. And for the specific case of US citizens abroad, the US citizen moving abroad guide and the FEIE vs FTC breakdown are the direct next reads. None of this substitutes for a qualified cross-border tax adviser before a move is finalised.