The teenager was detained after officers executed a warrant in Thornton Heath at about 00:05 BST on Thursday.
It takes 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, injuring 30 people.
Police have begun a search of the property where the teenager was arrested under Section 41 of the Terrorism Act.
Commander Dean Haydon, head of Scotland Yard's Counter Terrorism Command, said: This continues to be a fast-moving investigation.
"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 detained in Hounslow, west London, and an 18-year-old man was at the Port of Dover.
A time limit on Mr Farroukh's detention is expected to expire on Thursday, while detectives have been given until Saturday to continue questioning the 18-year-old.
