FINA, the sport's world governing body, said Efimova tested positive for 7-keto DHEA while she was training in Los Angeles in October, RIA Novosti reported.
She is suspended pending a FINA hearing.
The positive test could have been inadvertently caused by medication, said Russian Sports Minister Vitaly Mutko.
Efimova, 21, has three world championship gold medals and a bronze in the 200-meter breaststroke from the 2012 London Olympics. In December, she was named Russia's swimmer of the year.
FINA also reported another Russian swimmer, Sergei Makov, had been suspended.
Efimova is the fourth member of the 2012 Russian swim team to fail a doping test since the London Games.