When do I become a Portuguese tax resident?

Short answer: After 183 days physical presence in Portugal in any 12-month window, OR earlier if Portugal becomes your habitual residence.

Tax residency triggers worldwide-income taxation. The 183-day rule is the bright line, but having a registered address plus family in Portugal can pull the trigger earlier under the "habitual residence" test (Art. 16 CIRS). Plan with a tax advisor — find one in Partners.