Mark Anthony Peter Phillips (born 22 September 1948) is an English Olympic gold medal-winning horseman for Great Britain and the first husband of Anne, Princess Royal, with whom he has two children. He remains a leading figure in British equestrian circles, a noted eventing course designer, and a columnist for Horse & Hound magazine.

Mark Anthony Peter Phillips was born on 22 September 1948. He is the son of Major Peter William Garside Phillips, MC (1920–1998) and Anne Patricia Phillips (née Tiarks; 1925–1988); they married in 1946. Major Peter Phillips was the younger son of Joseph Herbert Phillips and grandson of William Garside Phillips. Anne Phillips was educated at Downe House and served in the Women's Royal Naval Service during the Second World War. She was the only daughter of John Gerhard Edward Tiarks (1896–1962), who served in the First and Second World Wars, attained the rank of Brigadier, and Evelyn Florence Cripps (1899–1994); John Tiarks also served as aide-de-camp to King George VI from 1947 to 1950. Mark had a younger sister, Sarah Anne Staples (née Phillips; 1951–2014).

Phillips was educated at Stouts Hill Preparatory School near Uley, Gloucestershire, then at Marlborough College, then the Royal Military Academy Sandhurst.

Upon passing out from Sandhurst, Phillips was commissioned as a Second Lieutenant into the 1st The Queen's Dragoon Guards in July 1969, he was promoted to Lieutenant in January 1971. By the time of his wedding to Princess Anne in November 1973, Phillips was an acting Captain. In January 1974, he was appointed a Personal aide-de-camp to Queen Elizabeth II. Phillips was substantively promoted to captain in July 1975, and retired from the Army on 30 March 1978.

Phillips continued to style himself Captain Mark Phillips, as it is usual for retired cavalry captains to keep using their former rank if their civilian job involves working with horses, in particular in racing or equestrian sports.

Phillips was a reserve member of the British equestrian team for the 1968 Olympics. He was part of the British three-day event teams that won a world title in 1970, a European title in 1971, and Olympic gold medals in 1972; individually, he finished in 35th place in 1972. At the 1988 Olympics, his horse sustained a pulled muscle and could not complete the individual three-day event, but Phillips won a silver medal with the British team. Phillips was a four-time champion at the Badminton Horse Trials, in 1971 and 1972 on Great Ovation, in 1974 on Colombus, and in 1981 on Lincoln. It was through his equestrian activities that he met Princess Anne, the only daughter of Queen Elizabeth II and Prince Philip, Duke of Edinburgh, whom he married. Their own daughter Zara Phillips later won a silver medal in the three-day event with the British team at the 2012 Olympics in London.

In 1998 Phillips designed the cross-country venue for the Red Hills Horse Trials, a qualifying event for the Olympics located in Tallahassee, Florida, United States. He is now a regular columnist for Horse & Hound magazine. He also remains a leading figure in British equestrian circles and serves as Chef d'Equipe of the United States Eventing Team.

In 2025 Phillips designed the cross-country course for 2025 European Eventing Championships.