He is the ninth person to be detained in connection with the attack, which killed 14 and wounded more than 50 earlier this month.
It was carried out by Kyrgyz-born Russian Akbarzhon Jalilov, 22.
What we know about the St Petersburg attack
In pictures: St Petersburg explosion
Abror Azimov is believed to have trained Jalilov ahead of the suicide bombing, Russia's Tass news agency reported.
He was carrying a pistol and a package containing two smartphones at the time of his arrest, reports said.
Jalilov is said to have detonated his device after a metro train left St Petersburg's Sennaya Ploshchad station, heading south for Tekhnologichesky Institut on the blue, north-south metro line.
Although the explosion happened in between the two stations the driver continued to Tekhnologichesky Institut to enable victims to be helped more easily.
Russia's investigative committee said this decision may have helped prevent further losses.
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