A police spokesman said the man was arrested in Stockwell, the south London neighbourhood where another suspect was detained on Friday and another man was shot dead by police in an underground station.
The first suspect, who has not been named, was being questioned at a high-security London police station.
Police said the second arrest occurred late on Friday and was “in connection with our inquiries” into Thursday’s attacks.
A huge manhunt has been mounted for the attackers who tried to bomb three underground trains and a bus, two weeks after similar bombings killed 52 people and four bombers.
The bombs in the latest attacks failed to detonate fully, and no one was injured.
On Friday, police released closed-circuit TV images of four suspected attackers and appealed to the public for information.