The police said on Sunday that a 21-year-old man was arrested late Saturday night in Hounslow in west London.
The man is being held under the Terrorism Act and questioned at a south London police station but the police has not charged or identified him yet.
An 18-year-old man was arrested on Saturday in the port area of Dover, where passenger ferries sail to France.
Two men are now being questioned for their possible roles in the attack that injured 29 people in London.
The arrests suggest security services believe the deadly incident at the Parsons Green station was part of a coordinated plot, not a lone wolf attack.
“We are still pursing numerous lines of enquiry and at a great pace,” counter-terrorism coordinator Neil Basu of the London police said late Saturday.
Prime Minister Theresa May put Britain on the highest security level of “critical” late on Friday following the bomb attack.
At “critical” level soldiers and armed police are deployed to secure strategic sites and hunt down the perpetrators of an imminent attack.
“For this period, military personnel will replace police officers on guard duties at certain protected sites,” May said in a televised statement.
The blast of a home-made bomb, which apparently failed to detonate fully, during the Friday morning rush hour on the west London tube was the fifth major terrorism attack in Britain this year.
The blast from the improvised explosive device caused burns for some passengers while others were injured in the blast’s stampede. Health officials said none of the injured were in serious condition.
The terrorist attack was claimed by Daesh Takfiri group, which is in war against the Iraqi and Syrian nations in the Middle East, but has also conducted attacks in other locations across the globe.
Daesh claimed other attacks in Britain this year, including two in London and one in Manchester.