The man was 60 years old on 26 June 2016 when he sexually assaulted an 11-year-old girl in Lae, Morobe Province.
He committed the act in the kitchen of his family home.
He pleaded guilty to one count of sexual penetration at the Lae National Court and was sentenced to 10 years in hard labour.
Five years of that sentence were suspended due to his age.
He was on bail prior to being sentenced by the court. That bail was forfeited and he will now serve a remaining balance of five years in jail.