The man, who has not been named, was arrested Saturday on suspicion of committing, preparing or instigating an act of terrorism after an attack in which 30 people were injured at Parsons Green station.
"We're keeping an open mind around whether more than one person is responsible for the attack," Neil Basu of London's Metropolitan Police told reporters.
Dover's ferry terminal was evacuated during the arrest and "a number of items" were recovered, while the teenager is now in custody in London.
The bomb detonated in a packed train carriage on Friday morning with a large explosion followed by what eyewitnesses described as a "fireball". It was Britain's fifth terror attack in six months.
Daesh claimed responsibility.