A 17-year-old man was arrested by police in Thornton Heath South London, less than ten miles from the site of the attack in Parsons Green which injured 30 people.
The midnight arrest took the total number of arrests in the investigation to six, all of whom remain in custody at a south London police station.
A homemade bomb partially exploded on a rush hour District Line train at Parsons Green. Many children were using the tube to get to school.
Police are searching the property where the teenager was arrested under the Terrorism Act.
Commander Dean Haydon, head of Scotland Yard's Counter Terrorism Command, said:
"A significant amount of activity has taken place since the attack on Friday.
"We now have six males in custody and searches are continuing at five addresses.
"Detectives are carrying out extensive inquiries to determine the full facts behind the attack."
Three men, aged 25, 30 and 48, have been arrested in Newport, south Wales. A 21-year-old - named as Yahyah Farroukh - was caught by police in west London and an 18-year-old man was at the Port of Dover, apparently on his way to Calais, France.