There has been no claim of responsibility for the attack but investigators say they are probing possible links to Islamic State jihadists. An explosion tore through a train as it was traveling between two stations in Russia's second-biggest city, injuring dozens more.
The suspect is the ninth person to have been detained on suspicion of having colluded with the bomber, Kyrgyzstan-born Akbarzhon Dzhalilov, since the attack on April 3.
The key suspect accused of plotting the St Petersburg metro bombing two weeks ago has been arrested near Moscow, BBC quoted Russian security officials as saying.
Eight people - all from mainly Muslim Central Asia - have been detained in Moscow and Saint Petersburg over alleged involvement in the attack.
The suspect has been taken for questioning to the Investigative Committee, which probes serious crimes, the FSB said.
The FSB said the man arrested on Monday is believed to have helped prepare the suicide bomber.
Azimov is believed to have trained the perpetrator of the bombing, Akbardzhon Dzhalilov, who left the explosive device in the train and was also killed in the explosion.
Seven other ethnic Central Asians have been held by police in both St Petersburg and Moscow.