Katinka Hosszu of Hungary won her second gold medal of the 2016 Olympic Games, after winning zero medals in her prior three appearances.
Hosszu’s time of 58.45 seconds was three tenths of a second ahead of Team USA’s Kathleen Baker, making her first Olympic appearance and winning the silver medal.
"I just was hoping I did well," Baker told media. "Going a best time and getting a silver medal is pretty incredible and I couldn’t be happier.”
Kylie Masse of Canada and Fu Yuanhui of China tied for the bronze medal with a time of 58.76.
American Olivia Smoliga placed sixth with 58.95.
Australian backstrokers Emily Seebohm and Madi Wilson took gold and silver, respectively, at the 2015 World Championships. They finished in seventh and eighth place Monday night.
Hosszu will also race the 200m IM semifinal about an hour after getting out of the pool from the 100m backstroke final.
2012 Olympic champion, Missy Franklin, did not qualify to defend his gold at the U.S. Trials in June.