Can I be self-employed and work as an employee at the same time?

Yes. It is perfectly possible: it is called pluriactivity (working both as a salaried employee and as a self-employed person).

What you need to take into account:

• You contribute to two schemes: to Social Security as an employee and also to the RETA as a self-employed person (although there are discounts in some cases of initial affiliation).
• Fiscally:
  • salaried work is taxed as earned income (payroll),
  • and self-employed activity as income from economic activities (invoices, VAT if applicable, instalment payments of Personal Income Tax, etc.).
• Income tax return: normally you will have to file it because you will have more than one type of income and you are also obliged to file it at all times, if you have an economic activity, regardless of income.
• Attention to compatibilities: if you are a civil servant or have contracts with exclusivity clauses, there may be limitations (contract/company must be reviewed).
• Unemployment: if you are receiving unemployment, becoming self-employed may affect the benefit (depending on the case).