IMSS NSS Number: How Foreign Residents Get or Find It Online
Employers ask for it on day one, and IMSS clinics, pension records and contribution weeks all run on it. How a foreign resident gets an NSS, finds a lost one and checks semanas cotizadas.
The NSS (número de seguridad social) is the personal number the Mexican Social Security Institute (IMSS) uses to track a person’s coverage and contributions. Foreign residents can get a new NSS or find an existing one online with their CURP and an email address. The procedure is free and the result is immediate, as of November 2026.
What is the NSS used for?
The NSS links you to the IMSS system for life. One person keeps one number, even when changing employers or moving between jobs and periods without work.
- Employment. Employers use it to register workers with IMSS. The Social Security Law (Ley del Seguro Social, Article 15) gives employers no more than five working days to report a new hire, a departure or a salary change.
- Medical care. Your registration at an IMSS family clinic (Unidad de Medicina Familiar) is tied to the NSS.
- Contribution weeks (semanas cotizadas). Your pension eligibility is counted in weeks recorded under your NSS.
- Voluntary coverage. People who enroll on their own, such as retirees, also need one. Our guide to IMSS voluntary enrollment for foreigners covers that route.
Having an NSS does not by itself give you medical coverage. Coverage depends on being registered by an employer or enrolled in a voluntary scheme and paid up.
Who needs to request an NSS?
Anyone who will be insured under IMSS needs one: employees, cooperative members, domestic workers and people who join voluntarily, such as independent workers. Most foreign residents first need it when a Mexican employer hires them or when they sign up for voluntary health insurance. If you worked in Mexico before, you probably already have a number, and the same procedure finds it.
What do you need to get an NSS online?
For the online route, IMSS asks for no documents, only two pieces of data:
- Your CURP (Clave Única de Registro de Población), the population registry code. Foreign residents get it with the resident card; see our page on what a CURP is and how foreigners get one.
- A valid email address, which IMSS links to your CURP.
The official gob.mx page for NSS assignment or lookup (homoclave IMSS-02-008) lists the procedure as available online 24 hours a day, every day, with an immediate response.
How do you get or find your NSS online?
- Go to the IMSS website and search for the procedure called Solicitud de asignación o localización de número de seguridad social.
- Enter your CURP and email address, and complete the security check.
- Open the email IMSS sends and follow the link to confirm.
- Download the document that shows your NSS. If you already had a number, the system returns it; if not, IMSS assigns a new one.
- Save the PDF and give the number to your employer.
If the online system cannot match your CURP, for example because your name is recorded differently, the request has to be done in person.
What if you have to go to an IMSS office?
In-person requests go to the IMSS affiliation office (Subdelegación or Oficina Auxiliar de Afiliación y Cobranza) for your address, Monday to Friday on IMSS working days, 8:00 am to 3:30 pm. The gob.mx page lists these originals:
| Document | What foreign residents can bring |
|---|---|
| Proof of identity in place of a birth certificate | An apostilled birth certificate with Spanish translation, or a naturalization letter or valid immigration document |
| CURP | A printout on plain paper counts as the original |
| Official photo ID | Valid passport, or valid immigration document such as the resident card |
| If someone files for you | Their official ID and a simple signed authorization letter (carta poder simple) |
IMSS’s contact center number is 800 623 2323, and IMSS keeps an online office directory.
How do you check your semanas cotizadas?
IMSS issues a contribution weeks certificate (constancia de semanas cotizadas, homoclave IMSS-02-025-A) online, free and immediately. You need your CURP, your NSS and an email address. On the IMSS site, search for “Semanas cotizadas”, choose the certificate, enter the data and download the PDF from the email link.
The certificate counts weeks up to your last departure or the date of the request. Check it after changing jobs: if an employer did not register you or reported a lower salary, the record will show it. Errors are corrected at the Subdelegación with a clarification request. IMSS also has a mobile app, IMSS Digital, that covers many of its online services.
Bottom line
Get your CURP first, then request the NSS online and keep the PDF with your residency papers. Download a contribution weeks certificate after each job change to confirm your employer registered you. For coverage questions, see our overview of IMSS, ISSSTE and IMSS-Bienestar.
FAQ
Can a foreigner get an NSS without a CURP?
The online route needs a CURP. In person, IMSS lists the CURP among the required originals, so get the CURP first through your resident card or INM.
Does the NSS change when I change employers?
No. The NSS stays the same for life. Each new employer registers you under the same number, and your weeks add up.
Is getting an NSS the same as having IMSS coverage?
No. The number only identifies you. Medical and pension coverage start when an employer registers you or you enroll and pay under a voluntary scheme.
Does the NSS cost anything?
No. The gob.mx page lists no fee for assigning or locating an NSS, online or in person, as of November 2026.
