Patrick Foster, 28, was arrested at his north London home early on Wednesday morning and is now being questioned by detectives.
He is being held on suspicion of computer-hacking offences and also conspiracy to pervert the course of justice over an alleged cover-up.
It is the 11th arrest carried out as part of Operation Tuleta, a Metropolitan Police investigation into alleged computer-hacking by journalists which is separate to the ongoing operations into phone-hacking and payments to public officials.
Mr Foster was a former graduate trainee on The Times who "outed" the secret police blogger Nightjack in an article published in 2009.
The author of the award-winning blog, named as Lancashire detective Richard Horton, failed in a legal bid to protect his anonymity.
Earlier this year the editor of The Times, James Harding, told the Leveson inquiry into press standards of "an incident where the newsroom was concerned that a reporter had gained unauthorised access to an email account".
He said: "The reporter believed he was seeking to gain information in the public interest but we took the view he had fallen short of what was expected of a Times journalist. He was issued with a formal written warning for professional misconduct."
The Metropolitan Police said in a statement on Wednesday: "Officers from Operation Tuleta, the investigation into criminal breaches of privacy including computer hacking which is being carried out in conjunction with MPS phone-hacking inquiries, arrested a man in North London this morning, 29 August.
"The 28-year-old man, a journalist (Tuleta arrest 11) was arrested at his home address at approximately 07.00hrs for suspected offences under the Computer Misuse Act 1990 and suspected conspiracy to pervert the course of justice, contrary to the Criminal Law Act 1977
"He is being questioned at a North London police station about alleged computer hacking relating to the identification of a previously anonymous blogger in 2009."
News International, publisher of The Times, declined to comment on Mr Foster's arrest.
Mr Foster has written articles on a freelance basis for The Guardian and The Daily Telegraph since being dismissed by The Times.
Meanwhile, a former editor of The News of the Word in Scotland, Bob Bird, has been detained by Strathclyde Police over allegations he attempted to pervert the course of justice in relation to the defamation action by former MSP Tommy Sheridan against the paper.