400 metres, Men

There was no single favorite in this event but two runners, Bahamian Steven Gardiner and American Michael Norman, were expecting to fight it out for the gold medal. The defending champion and world record holder Wayde van Niekerk (RSA) was also considered highly, but in October 2017 he had torn his anterior cruciate ligament playing football (soccer), and he was not back to his pre-injury level. Also expected to challenge for the medals were Grenadian Kirani James, the 2012 London gold medalist and 2011 World Champion, and the 2018 Commonwealth Games Champion, Isaac Makwala of Botswana.

All of the favorites save for van Niekerk made it to the final, where they were joined by another top American, Michael Cherry. James had been especially impressive in winning his semi-final in 43.88, as Gardiner led the third semi-final over Norman and Makwala, with van Niekerk trailing and not qualifying.

In the final James and Norman were out fastest with James holding a narrow lead at the halfway mark, as Gardiner was a good three metres back. At 300 metres, James still led but Gardiner had closed on the final turn and was close. Gardiner passed James on the straight to win the gold medal, as James was also passed by the Colombian Anthony Zambrano, who had been a silver medalist at the 2017 World Championships. Cherry finished fourth and Norman fifth as no Americans won a medal, unusual for the event, and Makwala was never a factor, placing seventh.

Gardiner won the first athletics gold medal for the Bahamas. For James, it was his third consecutive Olympics on the podium, completing his set of Olympic medals, after gold in 2012, and silver in 2016.