The man, aged 17, was detained under anti-terrorism powers in Thornton Heath, south of London, where a search is under way, a statement said.
Some 30 people were injured in the September 15 attack at Parsons Green station in west London, which was claimed by the Islamic State group.
Dean Haydon, an anti-terrorism officer, said the investigation was progressing rapidly.
The bomb had been hidden in a large white bucket and it apparently failed to detonate fully during morning rush hour but resulted in what witnesses described as a "fireball".
Some commuters suffered burns, while others were injured in the stampede to the exit that ensued.
The bombing was the fifth terror attack in Britain in six months, which combined have claimed 35 lives.
As part of the police investigation, an 18-year-old man, reportedly an Iraqi refugee, was arrested in the ferry departure terminal of the port of Dover on Saturday and a 21-year-old man was detained later the same day in west London.
The probe has focused on a foster home in Sunbury-on-Thames, a suburb of London where the 18-year-old is believed to have lived.
A third man, aged 25, was detained by anti-terror officers in Newport, Wales on Tuesday and two more arrests were made in the same city on Wednesday.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)
Get the latest election news, live updates and election schedule for Lok Sabha Elections 2019 on ndtv.com/elections. Like us on Facebook or follow us on Twitter and Instagram for updates from each of the 543 parliamentary seats for the 2019 Indian general elections.