Last week, a 25-year-old and another 30-year-old were arrested in Newport in Wales.
Police investigate after making arrests linked to the Parsons Green attack in Newport, South Wales, last week.
He has been taken to a south London police station where he is being quizzed by detectives.
So far, seven people have been arrested as part of the investigation being carried out by the Met's Counter Terrorism Command into the attack, which took place on September 15.
The man, 20, was detained at a house in the Welsh capital Cardiff by counter-terrorism officers, London police said.
Police last week charged 18-year-old Ahmed Hassan with planting the bomb at Parsons Green station on September 15.
UK Police Arrest Seventh Individual Over London Tube Terrorist Attack
Ahmed Hassan, 18, from Sunbury in Surrey, was charged with attempted murder by explosives.
He is said to have left the bucket bomb wrapped in a Lidl bag on a District line train and got off a stop before it exploded earlier this month.
Three others have been released without facing charges.
The attack was claimed by the Islamic State extremist group, but police cautioned that such claims were "routine... whether or not they've had any previous engagement with the individuals involved".
Two properties are now being searched: one in Surrey, and one in Cardiff.