She was arrested on suspicion of conspiracy to corrupt and suspicion of conspiracy to cause misconduct in a public office.
Ms Hartley is a long standing Sun reporter who has worked in Los Angeles for the paper and has also had spells as the consumer affairs correspondent and home affairs correspondent.
She was the 30th person to be arrested as part of Operation Elveden, which was set up to investigate allegations that journalists had illegally paid police officers and public officials for information and stories.
Police launched Operation Weeting, the inquiry devoted specifically to phone hacking, after receiving "significant new information" from News International on January 26 last year.
Operation Elveden was launched months later after officers were given documents suggesting News International journalists made illegal payments to police officers.
Officers also launched three related operations: the Sasha inquiry into allegations of perverting the course of justice; Kilo, an inquiry into police leaks; and Tuleta, the investigation into computer-related offences, as the inquiry escalated.
Metropolitan Police figures showed there were 829 potential victims of phone hacking, of whom 231 were said to be uncontactable.
The scandal has already led to the closure of the News of the World after 168 years, prompted a major public inquiry, and forced the resignation of Met Commissioner Sir Paul Stephenson and his assistant John Yates.