Efimova clocked a time of 29.78 seconds in the Barcelona pool to beat the previous best of 29.80 set by American Jessica Hardy in 2009.
Hardy was swimming alongside the Russian and posted the second-quickest qualifying time of 29.99 seconds.
Efimova, who won gold in the 50 breaststroke at the world championships in Rome in 2009 and silver in Shanghai two years ago, said she had barely slept following Friday's triumph.
"I still don't understand it," the still-dripping 21-year-old told reporters.
"Yesterday I was ready for a world record but today, I didn't sleep half the night and I am so tired," she added.
"I didn't do the warm-up and I am so sleepy and I just swam a record it's crazy."
She said she would try to go faster in Sunday's final.
"It's such a different technique (to the 200)," she added.
"I always like to swim the 200 but actually I am a short sprinter and for me it's like my birthday, it's like my holidays, for me it's always fun."
Efimova's world best in what is a non-Olympic event was the fourth of the week at the hilltop Palau Sant Jordi, all of them set by women.
Lithuanian teenager Ruta Meilutyte swam faster than anyone else before her in the 100 metres breaststroke semi-finals on Monday to beat another record previously held by Hardy.
American Katie Ledecky, like Meilutyte 16 years old, smashed the 1,500 metres mark to win gold on Tuesday and Denmark's Rikke Pedersen set a world best in qualifying for the 200 breaststroke on Thursday.
(Reporting by Iain Rogers; Editing by John O'Brien)
