After deliberating for more than three hours following the four-day trial, the Busselton District Court jury found 24-year-old Sione Vaioleti not guilty of aggravated sexual penetration without consent.
ABC Pacific reports the jury did, however, find him guilty of having sexual intercourse with a child under the age of 16.
One jury member cried as the verdict was handed down, while Vaioleti showed no emotion.
The lesser charge carries a maximum penalty of 14 years in prison, with Judge John Prior saying it was inevitable that Vaioleti would be jailed.
The 24-year-old was remanded in custody and was due to be sentenced on March 31.
Incident follows chance encounter
Assisted by a Tongan interpreter, Vaioleti had pleaded not guilty to aggravated sexual penetration without consent in the Busselton District Court on Monday.
In closing statements on Thursday, the jury was told Vaioleti, now 24, was with a friend about 8pm in late 2021 in the town's centre when he and the 15-year-old walked past each other, made eye contact, and smiled.
They saw each again other later that night and she asked Vaioleti if he had any cannabis.
The pair took a Snapchat video together, before going into an area behind a local bank.
State prosecutor Danielle Clarke told the court Vaioleti then sexually assaulted the girl despite her repeated refusals of his sexual advances.
Ms Clarke said the girl had called her father and friends, crying hysterically.
"Random opportunistic sexual offending does happen in this society," she said.
She alleged the encounter was not consensual.
"How many of you would want your first sexual experience to be in an alleyway, against a wet wall," Ms Clarke said.
"A 15-year-old girl, she had no intention to go down an alleyway with a larger man she's never met.'
Ms Clark said swabs from the victim revealed DNA from Vaioleti.
Defence argues it was consensual
Defence lawyer Michael Devlin said his client believed the encounter had been consensual and the girl told him she was 16 — although the prosecution told the court she said 15.
Mr Devlin said it was up to the state to prove that penetration had occurred.
He said Vaioleti had not "lured" the girl, and that the girl's consumption of alcohol that night might have contributed to her distressed response.
"It may have been a very unsatisfactory sexual encounter," Mr Delvin said.
The case was heard in the Busselton District Court.(ABC South West: Sam Bold)