According to Official Guinness Records,
The fastest long course 100 m backstroke by a female swimmer is 58.00 seconds, achieved by Kathleen Baker (USA) at the U.S. Swimming National Championships at the William Woollett Jr. Aquatics Center in Irvine, California, USA, on 28 July 2018.Baker was also a member of the US team that set a world record in the women’s 4 x 100 m medley relay at the 2017 FINA Swimming World Championships.
For a complete list of 2018 records, please visit 2018 Guinness Records in United States.