The 16-year-old boy was arrested on suspicion of Computer Misuse Act offences after detectives searched a Feltham address in west London, and later bailed, Scotland Yard said.
On Monday, a 15-year-old was arrested and bailed in Northern Ireland in connection with the "significant" attack on the TalkTalk telecom network's website last week.
The UK's leading phone and broadband provider, which has over four million UK customers, said banking details and personal information could have been accessed.
News of the hacking first emerged last Wednesday, with the telecom company publicly warning customers that their details could be used by fraudsters.
Chief executive Dido Harding later said the scale of the attack was "smaller" than originally thought, and that any credit card details accessed would have been incomplete.
British MPs are to launch an inquiry into the cyber-attack, with culture minister Ed Vaizey saying the government is not against compulsory encryption for firms holding customer data.