Written with people moving from Turkey in mind. The Portuguese rules still apply more broadly.
IRS Jovem: Tax Breaks for Youth
Young workers (up to 35 years old) with a bachelor's degree or higher can benefit from the "IRS Jovem" scheme. This offers a massive tax exemption on income for the first 5 years of your career in Portugal.
In the first year, you are 100% exempt from income tax (up to a limit). The exemption decreases gradually over the 5 years. You must explicitly select this option when filing your annual tax return.