Scott Chapman, 44, of Corby, was due for re-trial at the Old Bailey when he admitted the charge of misconduct in public office.
Chapman was jailed for three-and-a-half years on the same charge in 2014 before his conviction was quashed on appeal.
He was working at HMP Woodhill when he was paid Â£40,000 for 46 stories.
His conviction was quashed by the Court of Appeal when it ruled the Old Bailey judge at his first trial had "misdirected" the jury in the case against him and his co-accused, a former News of the World Reporter Lucy Panton and his ex-girlfriend Lynn Gaffney.
Their cases were later dropped following a Crown Prosecution Service review, but Chapman was ordered to face retrial.
The court heard he sold the stories to a range of newspapers, including the News of the World, between March 2010 and June 2011.
He is expected to be sentenced later and has been told to expect a jail term of two-and-half years, which would take into account the three-and-a-half months he served following his original conviction.
An ex-prison officer admitted selling stories to newspapers about James Bulger killer Jon Venables despite a previous conviction being quashed.