A 21-year-old man was arrested in the west London suburb of Hounslow in connection with the blast on a packed underground train at a London station on Friday.
He was detained under Britain's Terrorism Act and taken to a south London police station, said reports.
This comes close on the heels of the first arrest in this case when a 18-year-old was arrested on Saturday.
The Islamic State had earlier claimed responsibility for the improvised explosive device (IED) explosion which left 25 injured.
The explosion, on a District Line Tube in the British capital's southwest, occurred as the train pulled into Parsons Green station. Commuters said they heard a bang and saw a fireball inside the carriage when the "bucket bomb" exploded.
Several people suffered facial burns, some reportedly were even trampled in the rush to escape. But none of the injuries were life threatening. Pictures of a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor of the train went viral on social media.
Investigators were examining what appears to be a circuit board recovered from the scene, news reports said. Scotland Yard said it was a terror incident and they "assess" an IED triggered the blast.