Her visit to France comes after Prince George travelled to Marseille on Saturday 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 witnessed England beat Fiji 30-24 in the match on Sunday.
She was greeted by RFU president Rob Briers, and World Rugby chairman Sir Bill Beaumont, before being shown to her seat in the presidential box at Marseille’s Stade Velodrome.
Kate asked Sir Bill if he was “still surviving” after all the games so far to which he replied “Absolutely” adding that it had been “brilliant” and he had seen 18 matches.
Sir Bill then asked if George had enjoyed the match on Saturday to which she replied: “He did, he loved it so much.”
Speaking to Mr Briers, Kate, wearing a white jacket and black trousers, added: “Let’s keep our fingers crossed. I’ve been watching a few games. It’s been really great. It’s been a really successful World Cup.”
Kate, the patron of the Rugby Football Union, also attended England’s opening match of the tournament in September when they beat Argentina.
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.
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.