St. Petersburg Deputy Governor Anna Mityanina said on Twitter on April 21 that a woman injured in the blast died in the hospital. She did not name the victim.
Mityanina said 24 people injured in the attack, Russia's deadliest bombing since 2013, remain hospitalized.
Investigators say that 22-year-old Kyrgyz-born Russian citizen Akbarjon Jalilov, who was among the dead, carried out the attack.
Russian authorities have arrested 10 people of Central Asian origin on suspicion of involvement, and Federal Security Service (FSB) chief Aleksandr Bortnikov said on April 20 that officials have 'practically established' who ordered the attack.
Most of the 10 suspects deny involvement.
No group has claimed responsibility for the bombing.
With reporting by AFP