The Peter Moody-trained mare went off as the 1-6 favorite but only scraped home after coming under severe pressure from 5-1 shot Moonlight Cloud in a tense finish in front of Queen Elizabeth II and around 80,000 fans. Restiadargent finished third.
The 6-year-old Black Caviar is ranked second in the world among flatracing horses behind Frankel, who won the Queen Anne Stakes here by 11 lengths on Tuesday, and her mere presence created a real buzz at Ascot.
The Queen is celebrating her Diamond Jubilee to mark her 60-year reign.