The Women's 400 metres at the 2008 Summer Olympics took place on 16–19 August at the Beijing National Stadium. The winning margin was 0.07 seconds.

Sanya Richards-Ross is a retired American track and field athlete, who competed internationally for the United States in the 400-meter sprint. Her notable accolades in this event include being the 2012 Olympic champion, 2009 world champion, 2008 Olympic bronze medalist, and 2005 world silver medalist. With her victory in 2012, she became the second American woman to win the 400 meters at the Olympic Games and the first American woman to earn multiple global 400-meter titles. At this distance, Richards-Ross is also a six-time U.S. national champion.