McKeown's previous record of 26.86 was set by the five-time Olympic gold medallist at a World Cup meet in Budapest on October 20, 2023.
The record was the first of the 2026 championships.
"I knew I was capable of doing it but I didn’t expect it here, in this race, in this moment,” Curtis told
“I can tell that I am super woman right now, it’s unbelievable and I don’t think I’m going to be able to sleep tonight.”
The 19-year-old Italian swimmer, who studies at the University of Virginia, had won bronze in Tuesday's 100m freestyle but was top seed in the 50m backstroke after setting a European record 27.07 in the Sette Colli meeting in Rome last June.
She also took silver on Wednesday in the 4x100m freestyle.
Russian Alina Gaifutdinova was second fastest in the semi-finals with a time of 26.91 -- the third quickest time in history.
When McKeown set her time in 2023 she was the first person to hold the 50, 100 and 200m backstroke world records simultaneously since World Aquatics started keeping track of the shortest distance in 1997.
The Australian still holds the 200m backstroke record. She lost the 100m record to American Regan Smith in June 2024.
Sweden's triple Olympic champion Sarah Sjostrom won the women's 50m butterfly for a record-equalling 18th continental gold medal, a year after having her first child and after a 20-month absence from championship swimming.
Sjostrom, who holds the world record in the event, touched out in 25.05 seconds with Belgian Roos Vanotterdijk taking silver in 25.12, to tie German Franziska van Almsick's 18 European titles.
"It's great to be back at this level, I never thought it was possible a few months ago," the 32-year-old Swede said.
"I'm glad my body is still working. It's nothing I take for granted, so I'm happy with that."
Britain’s Angharad Evans won women’s 100m breaststroke gold in a championship record of one minute 04,87 seconds.