Can I work as an autónomo and also be a salaried employee at the same time?

Yes. Being both an autónomo and an employed worker (pluriactividad) is legally permitted in Spain. Both income streams must be declared in your annual Renta.

Under pluriactividad, Social Security will review your combined contributions (autónomo + employment) at year-end and refund any excess over the legal cap. If you stop your autónomo activity, remember to formally deregister (baja) to avoid continued autónomo obligations.