The royal cocker spaniel was taken to a field near Windsor to watch William and brother Harry wield their polo sticks for charity.
And he was clearly something of a novelty for guests. "Lupo seemed to get as much attention from the people at the presentation as William and Harry," an observer says.
Kate, 30, wearing a Libelula Studio shirt dress and Stuart Weitzman wedges, spent most of the game in or around a tent by the side of the polo field. She was accompanied by key aide Rebecca Deacon and was spotted playing with Lupo during the match.
Lupo seemed to relish the day in the spring sunshine. "Lupo was off his leash, running around during the polo game, and he found a friend – another dog – to play with too," the observer adds.
James Whatling / Splash News Online
At the end, when William and Harry were mingling with guests and present for the award-giving, Kate and Lupo joined them, and the lucky pup got some love from his two favorite princes.
"Spring feels it has finally started and Kate was enjoying the sunshine," the source, who watched the match, says.
For the princes, it was the second day of a weekend of polo matches played as the Audi Polo Challenge, which they do every year to raise funds for some of their charities.