Duncan John D'Arcy Armstrong OAM (born 7 April 1968) is an Australian former competitive swimmer, Olympic champion, and former world record-holder.

Convinced of his potential as a competitive swimmer, his family moved to Brisbane where he began training with the A.C.I.

[4] Armstrong made his international swimming debut in the 1986 Commonwealth Games in Edinburgh, Scotland.

He won his first gold medal in the 400-metre freestyle in dramatic fashion by surging from behind after trailing by nearly 25 metres at the midway point of the race.

[2] Armstrong earned his second gold medal as a member of the winning Australian team in the 4×200-metre freestyle relay.

[5] Armstrong arrived in Seoul for the 1988 Summer Olympics ranked 46th in the world,[2] facing a trio of past and current world record holders in the men's 200-metre freestyle: Matt Biondi of the United States, Artur Wojdat of Poland, and Michael Gross of West Germany.

[5] Although not favored to win, Armstrong had developed a strong sense of confidence after years of 20 km per day of training.

[11] Armstrong was also a member of the Australian team that finished fourth in the men's 4×200-metre freestyle relay.