You will need to apply for a taxpayer's number (NIF) before the Portuguese tax authorities. That, however, does not mean you will automatically transfer your tax residency to Portugal since you can apply for the taxpayer's number (NIF) as a non-resident. You can keep your tax residence outside Portugal if you don’t spend more than 183 days each year in Portugal. If this occurs, you will become a tax resident in Portugal. The main advantages of a non-resident regime are: special PIT (personal income tax) regime for a 10-year period; employment and self-employment income can be liable to a 20% flat rate if derived from high value-added activities of scientific, artistic or technical nature performed in Portugal; for example, industry CEOs, doctors, university teachers, engineers, journalists etc. On income from abroad, employment income can be exempt from PIT provided that it is taxed in the country of origin according to the applicable tax treaty or, if there is no applicable treaty, the income is effectively taxed in the country of origin and it is not deemed as derived in Portugal. Foreign source dividends, interest, capital gains and rental income, together with self-employment and professional income (only in case of high value-added activities), can be exempt from PIT if the income is liable to tax in the country of origin, according to the applicable tax treaty or the OECD Model Tax Convention, and also if it is not deemed derived in Portugal and also not deemed obtained in a tax haven.
