Tupou's plea was entered by his lawyer, Donald Kerslake, before District Court judge Alalatoa Rosella Papali'i.
During the proceedings, prosecuting lawyer Vaisala Afoa revealed that Tupou, who was suspended from all rugby for two weeks, had an outstanding warrant of arrest from 2016 for a separate charge of assault.
The Samoa Observer reports Judge Alalatoa granted an application by the defence counsel to cancel the warrant of arrest against Tupou and referred the matter to the Family and Violence Court.
Sentencing has been set for next Friday.
Last week, the Samoa Rugby Union suspended the 30-year-old Tupou for two weeks after he was stood down from the London and Paris legs of the World Sevens Series.