Swimming at the 2020 Summer Olympics – Women's 400 metre individual medley

The women's 400 metre individual medley event at the 2020 Summer Olympics was held on 24 and 25 July 2021 at the Tokyo Aquatics Centre. It was the event's fifteenth consecutive appearance, having been held at every edition since 1964.

Summary
Japan's home favourite Yui Ohashi held off a late charge from the U.S.' Emma Weyant to win her nation's first Olympic title in the event. Second at the halfway mark, Ohashi used a stunning breaststroke leg to separate herself from the tight field and touch in 4:32.08 for gold. Meanwhile, Weyant, 1.99 seconds behind Ohashi heading into the freestyle, stormed home to take silver in 4:32.76. Weyant's teammate Hali Flickinger moved through the field in the final lap to win bronze more than two seconds behind in 4:34.90.

Spain's defending bronze medallist Mireia Belmonte (4:35.13) could not repeat her podium efforts from Rio five years earlier and settled for fourth. In the hunt for a medal, Hungary's defending champion Katinka Hosszú faded down the stretch to take fifth in 4:35.98, almost 10 seconds off her world record set at the last Games. Hosszú's teammate Viktória Mihályvári-Farkas (4:37.75) took sixth, while Great Britain's Aimee Willmott (4:38.30) repeated her seventh-place finish from Rio five years earlier. Outside the sub 4:40 club, Italy's Ilaria Cusinato (4:40.65) rounded out the championship field.

Records
Prior to this competition, the existing world and Olympic records were as follows.

No new records were set during the competition.

Qualification
The Olympic Qualifying Time for the event is 4:38.53. Up to two swimmers per National Olympic Committee (NOC) can automatically qualify by swimming that time at an approved qualification event. The Olympic Selection Time is 4:46.89. Up to one swimmer per NOC meeting that time is eligible for selection, allocated by world ranking until the maximum quota for all swimming events is reached. NOCs without a female swimmer qualified in any event can also use their universality place.

Competition format
The competition consists of two rounds: heats and a final. The swimmers with the best 8 times in the heats advance to the final. Swim-offs are used as necessary to break ties for advancement to the next round.

Schedule
All times are Japan Standard Time (UTC+9)

Heats
The swimmers with the top 8 times, regardless of heat, advance to the final.