The Duchess of Cambridge's socially ambitious little sister travelled to Gerpinnes, south of Charleroi, for the December 1 event, arranged by the family of Belgian billionaire Albert, Baron Frere, according to local reports.
Pippa is said to have been among the 20-strong party who set out from the Bierlere forest near Gerpinnes. Six boars and nine deer were killed in the hunt, the Daily mail reported.
Animal rights charity 'Viva' rapped her for going on last month's hunt, which has only just emerged.
"It's a shame she's so desperate to be royal by association that she's now seemingly aping their hunting bloodlust. There is nothing classy about shooting terrified wild animals for sport," spokesman Justin Kerswell was quoted by the Sun as saying.
The League Against Cruel Sports added: "It's sad to see someone in such an influential position indulging in this cruelty".
The Frere family, who are akin to royalty in their home country, refused to comment, saying "hunting is a private affair".
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But three anonymous sources in the hunting party confirmed 29-year-old Pippa's presence at the event, Belgian newspaper La Capital reported.
"I shook her hand but at that moment I didn't recognise her," said one. "Afterwards a friend teased me, saying: what, you didn't recognise Pippa?"
Another hunting party member said he didn't realise Pippa was Kate's sister because she introduced herself by her full name, Philippa.
The hunt, one of several put on each year by the family, was organised by Albert Frere's son, Gerald, and his two grandsons, 27-year-old William and Cedric, 28.