Larrabee is from the Twin Falls area in Idaho.
The high court's decision sends the case back to District Court for a new trial.
Larrabee was found guilty of two counts of sexual abuse involving a family relative and one count of dealing in material harmful to a minor. The case dates back nearly 10 years.
But The Spectrum of St. George reports justices determined Deputy Washington County Attorney Ryan Shaum prejudiced the jury by introducing the possibility that Larrabee might have been accused of sexual abuse by another person.
Justices say the court had barred any discussion of the claim against Larrabee by a family member whose account was judged to be "unreliable," but Shaum's comments appeared to violate that order.