The girl's mother reported the father for alleged sexual abuse of her daughter, who initially said she had been raped in the street before changing her story to corroborate her stepfather's claim he had not been conscious when they had sex.
The judges, sitting in the same Pamplona court that in 2018 sparked massive protests when acquitting the "wolf pack" gang rapists who were later found guilty on appeal, said that there was insufficient evidence to convict the stepfather.
However, the sentence also noted the girl had "kept her head and eyes facing down" during the trial, "answering questions very briefly in a barely audible voice".
Altamira Gonzalo, vice-president of the Themis women jurists' association, said the verdict "takes us back to the kind of sentences we used to see in Spain in the 1950s or 60s, when there was complete impunity for men in the family environment".
"The sentence beggars belief. The only thing the court valued is the accused's right to the presumption of innocence. It has ignored its duty to protect a minor and the fact that the age of consent in Spain is 16," Gonzalo told The Telegraph.
Spain's parliament this week passed a new child protection law backed by NGOs such as Save the Children, which estimates only 15 per cent of family abuse cases in Spain are reported.
Cira Garcia, a judge from one of Spain's gender violence courts, said the sentence does not even consider the possibility the girl may have changed her version of events in order to protect her stepfather.
"We are sick of criticising that children's testimony is often not given credibility and they are accused of lying in court, but here the girl's word is accepted without question to absolve the accused, even though her account of events is absolutely implausible," Garcia told the online newspaper Publico.
SEXUAL HARM - DO YOU NEED HELP?
If it's an emergency and you feel that you or someone else is at risk, call 111.
If you've ever experienced sexual assault or abuse and need to talk to someone contact the Safe to Talk confidential crisis helpline on:
• Text 4334 and they will respond
• Email support@safetotalk.nz
• Visit https://safetotalk.nz/contact-us/ for an online chat
Alternatively contact your local police station - click here for a list.
If you have been abused, remember it's not your fault.