A Federal Prison Service representative said Nadezhda Tolokonnikova was taken to the hospital at her own and her lawyer's request, RIA Novosti reported.
The representative did not elaborate, citing patient privacy.
Yekaterina Samutsevich, another Pussy Riot member, told a Russian television station Tolokonnikova has been suffering from headaches.
Tolokonnikova, 23, and another Pussy Riot member Maria Alyokhina, 24, were each sentenced to two years in prison for "hooliganism motivated by religious hatred" for performing an anti-Kremlin "punk prayer" at Moscow's Christ the Savior Cathedral in February 2012.
Samutsevich, who received a suspended sentence, said Tolokonnikova's health problems began before she was convicted in August.