Police raided two homes in western London after the man was detained at 11:40 a.m., according to London's Metropolitan police.
The man was held in Kensington Church Street, west London by officers from Scotland Yard's Anti-Terrorist Branch.
He was arrested on suspicion of the commission, preparation or instigation of an act of terrorism under the Terrorism Act 2000 and taken to Paddington Green police station in London for questioning.
The spokesman said he is the 44th person to be arrested in connection with the failed attacks.
"He has been taken to a central London police station where he will be interviewed," the spokesman said.
The suspect was detained for allegedly supporting the attackers after the bombing.
Sixteen people have been charged with offenses in relation to the July 21 attacks.
Five men are accused of conspiracy to murder and are due to stand trial at London's Central Criminal Court in September.
A sixth man is accused of conspiracy to cause explosions while 10 others are charged with failing to disclose information. No date has been set for their trials.
Last month, British police investigating the failed July 21 bombings charged a student with conspiracy to cause explosions, authorities said.
London's Metropolitan Police identified the man as Adel Yahya, 23. He was arrested Tuesday at Gatwick Airport as he got off a flight from Addis Ababa, Ethiopia.
He was charged with conspiring with four other men, all of whom are awaiting trial over the plot to attack three subway trains and a double-decker bus "to cause by an explosive substance, explosions of a nature likely to endanger life or cause serious injury to property."
Yahya, from the Tottenham area of north London, is scheduled to appear at Bow Street Magistrates' Court on Friday.
A total of 16 people have now been charged in connection with the attempted attacks, which did not kill anyone as the bombs failed to detonate.
But the news shook Britain's capital two weeks after near identical suicide bombings killed 52 commuters on July 7.
Five men are accused of plotting to murder passengers on London's transit system on July 21 and face trial in September.
Muktar Said Ibrahim, 27, Ramzi Mohamed, 23, Yassin Omar, 24, Hussein Osman, 27, are accused of trying to bomb three subway trains and a bus. The fifth suspect, Manfo Kwaku Asiedu, 32, was arrested after a backpack of explosives was found in a raid. All face a charge of conspiracy to murder.
after the attacks and was extradited back to Britain. Osman has said through his lawyer that the bombing attempt was meant to scare people, not kill them.
Yahya is accused of conspiring with Osman, Ibrahim, Asiedu and Omar.
Ten people have been charged in connection with the attempted attacks for failing to disclose information about the suspects and helping them evade arrest. They include Osman's wife Yeshiemebet Girma, 29, her younger sister Mulumebet Girma, 21, and their brother Essayas Girma, 20