The FSB said it detained the man -- named as Abror Azimov -- today in the Odintsovo suburb of Moscow, describing him as "originally from Central Asia," in a statement quoted by RIA Novosti news agency.
The suspect has been taken for questioning to the Investigative Committee, which probes serious crimes, the FSB said.
The suspected suicide bomber has been named as 22-year- old Akbarjon Djalilov, who is thought to have been a Russian national born in Central Asian Kyrgyzstan.
Djalilov is suspected of triggering a homemade explosive device while on a train in a metro tunnel. Security forces said they safely defused another device left on a platform.
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Eight were previously been detained in Moscow and Saint Petersburg over the bombing, with the head of the FSB Alexander Bortnikov saying last week they all came from Central Asia.
The attack has posed tough security questions as Russia gears up to host the opening game and final of the Confederations Cup football tournament in June, ahead of the World Cup in 2018.
The Kremlin has called the attack a "challenge to every Russian, including the head of state".