Fa'amausili Moli Malietoa was also found guilty by a district court judge of uttering insults and object throwing.
Fa'amausili is the son of Malietoa Tanumafili II.
The Samoa Observer reported the verdict followed a trial held earlier this year and the charges relate to a family dispute.
Fa'amausili originally faced four charges, but the charge of assault was dismissed.
Sentencing for the matter has been scheduled for 24 May.
Last December the Land and Titles Court ruled in favour of Fa'amausili getting the Malietoa title.