Former police sergeant James Bowes pleaded guilty at the Old Bailey on Friday to misconduct in public office, admitting he sold information to The Sun newspaper between April and July 2010.
Bowes could receive a jail sentence for the crime and will be sentenced on 9 May. He was granted unconditional bail but warned it was "no indication of disposal". No further details of the case were revealed at the hearing.
Deputy chief constable of Sussex Police, Giles York, said in a statement: "On being made aware of the investigation into James Bowes in August 2012, Sussex Police immediately suspended him and following an internal disciplinary hearing the following month, he was dismissed for gross misconduct."
Bowes contacted The Sun in April 2010 offering to sell information about police cases. He was paid £500 after the publication of an article. He is one of more than 60 people to be arrested under Operation Elveden, an investigation into payments made to public officials to gain information by the press.
Ex-Surrey PC Alan Tierney and ex-prison officer Richard Trunkfield received jail sentences last month for selling information to The Sun.
Police are continuing work on a number of investigations into alleged illegal practices by journalists to obtain information, including phone hacking.