Her visit to France comes after Prince George travelled to Marseille with his father the Prince of Wales to cheer on Wales, but saw the team’s dream of World Cup victory come to an end with defeat by Argentina.
The Princess will watch from the stands on Sunday with hopes of seeing England advance to the semi-final against France or South Africa.
Kate, the patron of the Rugby Football Union, attended England’s opening match of the tournament in September when they beat Argentina.
There she was greeted by RFU president Rob Briers, France 2023 chairman Jacques Rivoal and World Rugby chairman Sir Bill Beaumont before being shown to her seat in the Presidential Box at Marseille’s Stade Velodrome.
She spoke with each of the administrators in turn but spent longest with Sir Bill, the former England captain.
“We’re really excited to be here. Shame we can’t be here longer,” she said.
Ten-year-old George joined his father at the Stade de Marseille for the first international sporting fixture he has watched in person overseas.
The pair were greeted by Sir Bill, France 2023 deputy chief executive Martine Nemecek and Welsh Rugby Union president Gerald Davies.
George was the first to be welcomed by Sir Bill, stepping forward to shake the former England player’s hand and say hello.
Introducing his son to Mr Davies, William remarked: “Gerald wore the jersey for Wales and was one of the best players ever. Bill played for England.”
Pointing to Mr Davies and then to Sir Bill, William joked: “He was very fast and he was quite sluggish.”
Prince George, who is gearing up for school entrance exams this autumn, plays rugby at school.
England finished on the top of their pool, winning all four of their games, while Fiji finished second in their pool, beating Australia and Georgia, but narrowly losing to Wales and Portugal.
Fiji beat England in late August 30-22 at Twickenham in August in a warm-up game for the tournament.