Nadeem threw an Olympic record of 92.97 metres for victory, India's defending champion Neeraj Chopra taking silver with 89.45. Grenada's Anderson Peters claimed bronze with 88.54m.
Before Arshad's remarkable victory, Pakistan had never won an individual gold medal at the Olympics.
All of Pakistan's previous three gold medals came in field hockey, with their team winning gold in 1960, 1968 and 1984.
Prior to Thursday, only two Pakistan athletes had won individual medals of any colour – with a wrestling bronze in 1960 and a boxing bronze in 1988.
Since the 1992 Barcelona Games, Pakistan has not won a medal of any kind.
