Craig S., a soldier formerly with the 7th Parachute Regiment Royal Horse Artillery, assaulted the girl in her bedroom after sneaking into her home in the Stubai Valley.
The girl's father woke to find a naked man attacking his daughter sexually.
He told the court "I was half asleep and first thought it was my wife but then I realised she was lying next to me.
I then saw the naked stranger in the room with my daughter. I completely flipped out and hit him in the face several times."
The defendant told the court he'd had 'quite a few' drinks with his friends, and claimed to have no memory of the incident. He explained that he had fought in war zones, and was suffering from PTSD.
"There is no explanation for what I did, and there is also no excuse."
A psychiatrist appointed by the court, Reinhard Haller, confirmed the diagnosis but also stated that he could find no signs of insanity.
The defendant's lawyer said "The combination of alcohol and trauma played a major role."
"In spite of this, he had not previously shown any such inclinations, making his behaviour inexplicable."
The attack took place in November 2014. The man has been held in investigative custody in Innsbruck since his arrest.