Married father-of-two David Black, 52, was shot several times from a car that pulled up alongside his on the M1 near Lurgan, Co Armagh as he drove to Maghaberry jail.
Colleagues have said Mr Black, from Cookstown, Co Tyrone, had been considering retirement after more than 30 years' service.
Prime Minister David Cameron joined political leaders on both sides of the Irish border in condemning a "brutal murder". He said: "These killers will not succeed in denying the people of Northern Ireland the peaceful, shared future they so desperately want."
Stormont First Minister Peter Robinson branded the culprits "flat-earth fanatics living in the dark ages, spewing out hatred from every pore".
After being shot, Mr Black's Audi A4 veered off the road and crashed into a deep drainage ditch.
Police have blamed dissident republicans opposed to the peace process. The violent extremists have been engaged in a long running protest campaign against conditions inside HMP Maghaberry in Co Antrim – Northern Ireland's only maximum security prison.
Stormont Deputy First Minister Martin McGuinness insisted the murder would not destabilise the peace process. "Our community stands absolutely four-square and united against the activities of these groups," he said.
Mr Black is the 30th prison officer killed in Northern Ireland since 1974, though the first for almost 20 years.
Police Service of Northern Ireland Assistant Chief Constable Drew Harris said that dissidents had been actively targeting prison officers. He said: "The motive behind this is sheer terror."