Written with people moving from Turkey in mind. The Portuguese rules still apply more broadly.
Unmarried Couples (União de Facto)
If you have lived with your partner for over 2 years, Portugal recognizes your relationship as a "União de Facto". This grants you similar rights to marriage regarding taxes (IRS), inheritance, and social security.
To prove this status for visa or tax purposes, you usually need a declaration from your local Junta de Freguesia stating that you reside at the same address. It is a valid route for family reunification without a marriage certificate.