Argentina's Mateo Carreras scored a hat-trick to help the Los Pumas see off Japan 39-27 in their winner-takes-all Pool D clash on Sunday.
England qualified for the quarter-finals of the World Cup after Japan saw off 14-man Samoa 28-22 in Toulouse to keep alive their own hopes.
Billy Vunipola has been banned for the start of England's World Cup campaign after receiving a three-week suspension for his red card against Ireland.
Fiji secured their sixth Pacific Nations Cup title after they beat Japan 35-12 in their final-round match at Prince Chichibu Memorial Stadium on Saturday.
Samoa defeated Japan 24-22 in their Pacific Nations Cup opener in Sapporo after Michael Leitch's red card in the 30th minute.
Japan head coach Jamie Joseph believes that their unique style will cause their Pool D rivals problems at the Rugby World Cup.