The NISS is Portugal's social security identification number. It is separate from the tax number, the NIF.
What it is
The NISS identifies a person within Portugal's social security system. It is issued by the Segurança Social, also called the ISS (the social security institute), and is separate from any tax number a person holds.
What it is for
The NISS is used to register a person for social security contributions and benefits. Employers use it to report and pay contributions for their employees. Self-employed workers use their own NISS to pay their own contributions and to build up entitlement to future benefits, such as sickness or retirement benefits.
Who needs it?
A person who works in Portugal and is covered by the Portuguese social security system needs a NISS, whether as an employee or as a self-employed worker. It applies to people already living in Portugal and to people coming from another country to work there. A worker who stays covered by the social security system of another country, for example under an A1 certificate, may not hold a NISS. Employers on a construction site have to record the NISS of their workers there (Decree-Law 273/2003, article 21), so it is worth clarifying the position early.
How it relates to the NIF
The official NISS application asks for the NIF if one has already been assigned, and eligible applicants can request the identification numbers together. Getting a NIF first is therefore not a universal rule. Having a NIF does not automatically mean a NISS has also been set up, and the reverse is also possible. More info can be found at the Segurança Social website.
What to watch out for
It is worth checking, calmly and factually, that someone with a NIF also has a NISS where one is expected. A person can have a valid NIF and still not be registered with social security. Collecting a NISS does not by itself decide which country receives the person's social security contributions.
Having a NIF does not automatically mean a NISS exists as well. Check both where both are expected, and do not assume that holding a NISS settles which country's contributions apply.