The 21-year-old was detained just before midnight on Saturday
British police and forensic teams are searching a property in Surrey following the arrest of a second man by officers investigating a terror attack on the London Underground.
The 21-year-old was detained in the Hounslow area of London at around 11.50pm on Saturday.
He is being held at a police station in the British capital under the UK's Terrorism Act.
It comes after an 18-year-old man was arrested on Saturday in the departure lounge of Dover port in connection with the blast.
Thirty people were injured on Friday when an improvised explosive device detonated on a Tube train at Parsons Green.
Officers have also searched a house in Surrey they said was directly linked to the first arrest.
The property is registered to an elderly foster couple who had been honoured for their work with child refugees.
The terror threat level in Britain has been lowered from 'critical' to 'severe' following the recent arrests.
It means a terror attack is "highly likely" there.
In the immediate aftermath, the threat level was temporarily raised to its highest level, triggering military deployment to free up armed police to patrol the streets.
But on Sunday, the British Home Secretary Amber Rudd announced it was being downgraded.
"I would urge everybody to continue to be vigilant but not alarmed," she said.
Ms Rudd also directly criticised US President Donald Trump over his claim the bombers were known to Scotland Yard.
Dismissing it as "speculation - not a leak", she told the BBC's The Andrew Marr Show: "It is never helpful to have speculation about an ongoing operation and I would include the President of the United States in that."
In the wake of the bombing, police hinted they were searching for more than one suspect, and said they were keeping an "open mind" as to whether multiple people were involved.