AUG (Authorized User Group) Certified
IND Recognised Sponsor
Nasscom Certified
SNA Certified
Nasscom Certified
What Is the National Social Security Office (NSSO)?
The National Social Security Office (NSSO), known as ONSS in French and RSZ in Dutch, is the Belgian government body that collects social security contributions from employers and employees and administers Belgium's social insurance system. Every company hiring staff in Belgium must register with the NSSO before making its first hire.
Employee social security contributions in Belgium are a fixed 13.07% of gross salary, with no income ceiling, while employer contributions vary by worker category and are calculated as a percentage on top.
NSSO at a Glance
| Attribute | Description |
|---|---|
| French / Dutch Name | ONSS / RSZ |
| Employee Contribution | Fixed 13.07% of gross salary |
| Employer Contribution (2026) | Roughly 27% for white-collar, ~33% for blue-collar employees |
| High-Earner Relief | Basic employer contribution exempted above €85,000/quarter (through 2026) |
| Reporting Mechanism | Quarterly DmfA declaration |
| Best For | Understanding the true cost of Belgian social security compliance |
Why Does It Matter?
Belgian social security contributions represent one of the highest employer costs in Europe, materially affecting the true cost of hiring. Alongside Belgian payroll tax withholding, every employer must register with the NSSO, file quarterly declarations, and remit both employer and employee contributions accurately. Missteps here are a common and costly compliance failure for companies new to the Belgian market.
When Is It Used?
NSSO registration and reporting become relevant whenever a company:
- Hires its first employee in Belgium and must register as an employer.
- Runs payroll and needs to calculate and remit both employer and employee contributions.
- Files the mandatory quarterly DmfA declaration reporting wages and hours worked.
A US software company hires its first employee in Brussels at €5,000 gross monthly salary. Before the employee's start date, the company registers with the NSSO and files a Dimona declaration, then each pay period withholds the employee's 13.07% contribution and separately pays its own employer contribution of roughly 27%, filing the details through the quarterly DmfA declaration.
Common Misconceptions
No. Employer contributions differ based on whether the employee is classified as white-collar or blue-collar, among other factors.
No. The 13.07% employee contribution applies to the full gross salary with no ceiling, unlike many other European systems.
No. It only exempts the basic ~25% contribution above a quarterly salary threshold; the additional ~3% and the full employee contribution still apply.
No. The employer itself must be registered with the NSSO; a payroll provider or social secretariat handles filings but doesn't replace this registration.
The NSSO is the central authority overseeing Belgian social security contributions, collecting a fixed employee rate and a variable employer rate that together add significantly to the cost of employment. Companies hiring in Belgium need to register early and build accurate contribution calculations into their payroll from day one.
Hiring in Belgium?
We'll handle your NSSO registration, contribution calculations, and quarterly DmfA filings from day one.
experts@adtsolution.com
Queens towers, Delflandlaan 1, 1062 EA Amsterdam, The Netherlands
