The attacks happened on the Balearic Islands, a popular vacation resort, between June and July. There was no evidence of related robbery or sexual abuse.
The attacks follow a string of cases of young women being pricked with syringes or injected with toxic substances in crowded space across Spain, France and the United Kingdom.
No traces of drugs or other toxic chemicals were reported in the attacks on four women in Ibiza and two in Mallorca, the Spanish news agency said.
Similar cases were registered at the bull-running festival in Pamplona in Navarre province, in Catalonia's Barcelona, and in the Basque County. Traces of a toxic substance were found in one case.