A 23-year-old prison officer has been arrested on suspicion of attempting to smuggle drugs into Maghaberry Prison.
He was arrested at the prison at about 10:20 BST on Friday.
Two women, aged 55 and 26, were also arrested on suspicion of smuggling drugs into prison following police searches in properties in Lisburn and Dungannon.
The 26-year-old woman has since been released on bail pending further enquiries.
Police said Class A, B and prescription drugs were recovered.
The officer is being questioned on suspicion of misconduct in public office, possession of criminal property, and the supply of drugs.
Head of the Northern Ireland Prison Service, Ronnie Armour said: "I welcome the result of today's joint operation with the Police Service of Northern Ireland.
"Removing dangerous drugs from Maghaberry will help to keep staff and prisoners safe."
Detective Chief Inspector Brian Foster, from the PSNI's Criminal Investigation Branch described the operation as a "good example of how collaborative working can disrupt crime".
He said: "We will continue to work closely with the Prison Service to clamp down on the trafficking of illicit drugs into prisons."