A prisoner let out on temporary release is now being treated as "unlawfully at large".
William Patterson is serving time for a series of crimes including grievous bodily harm (GBH) and robbery and was due back at a Derry jail on March 20.
Police are now working to locate the 37-year-old whom they said has connections in Belfast and Dundonald, Belfast Live reports.
A PSNI spokesperson said: "Patterson, who was serving a sentence for four counts of grievous bodily harm with intent, grievous bodily harm, six counts of theft, criminal damage and two counts of robbery was temporarily released from prison on 18 March and was due to return on 20 March.
"Patterson has so far failed to return."
The prisoner was being held at HMP Magilligan in Derry.
They added: "He is described as being around 176cm tall, of slim build with brown hair and a fresh complexion. We believe he has connections to Belfast and Dundonald.
"Despite exhaustive attempts to arrest Patterson, police have been unable to locate him. We are appealing to anyone who has seen William Patterson, or who knows of his whereabouts not to approach him but to contact police on 101 quoting reference number RM20022657.
