The NISS is Portugal's social security identification number, and a person needs a NIF before they can get one.
What it is
The NISS (social security identification number) 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
Anyone working in Portugal, as an employee or as a self-employed worker, needs a NISS. It applies to people already living in Portugal and to people coming from another country to work there, whatever the length of their stay. A site worker moving between short jobs still keeps the same NISS throughout.
How it relates to the NIF
A NIF (the personal tax number) is needed before a person can get a NISS. The two numbers are linked, since social security registration builds on a person's existing tax identification. Having a NIF does not automatically mean a NISS has also been set up, though the two are normally requested close together in practice, one after the other.
What to watch out for
It is worth checking, calmly and factually, that someone with a NIF also has a NISS. A person can have a valid NIF and still not be properly registered with social security. This is worth double-checking rather than assuming that having one number means the other one exists too.
Having a NIF does not automatically mean a NISS exists as well. It is worth checking both, rather than assuming one covers the other.