Published Thursday, 17 June 2010
In a statement he said he has informed Justice Minister David Ford of his decision.
Mr Masefield said: "I had originally 'signed up' for five years from my appointment in December 2004, but was pleased to stay on to cover the transition at devolution.
"It has been a privilege to serve in this capacity and while it has been challenging and demanding, it has also been rewarding."
"I look forward to continuing to serve with the Prison Service for the remainder of this period, as there remains much important work to be done."
While indicating he is leaving on his own terms, Mr Masefield has faced down earlier calls to quit his £100,000 a year role.
His period at the helm, particularly the latter years, saw the NIPS rocked by a number of damning reports related to inmate suicides.
Criticism of Maghaberry
The most critical focused on the death of 34-year-old convicted murderer Colin Bell, who hanged himself in the top security Maghaberry jail near Antrim in 2008 despite being on 24-hour watch.
Investigators found that wardens who were supposed to be checking the high-risk inmate were lying on beds watching TV.
That prompted the removal of Maghaberry's governor and deputy governor and the disciplining of a number of staff.
But only weeks after new governor Steve Rodford took charge another prisoner killed himself inside Maghaberry.
Four prison medics were suspended from duty following a probe into the suicide of convicted sex offender John Anthony Deery, 50, who hanged himself in August 2009.
Then Mr Rodford, who was tasked with reforming the regime and improving prisoner monitoring inside Maghaberry, left his post after only five months in office over fears that he would be targeted by dissident republicans.
Earlier this year the body representing rank and file staff - the Prison Officers' Association - called for Mr Masefield and other members of the NIPS management team to quit over their handling of the disciplinary procedures against its members in the wake of the Colin Bell death.
On top of those problems, Maghaberry has been severely criticised in recent times by dissident republican prisoners who claim mistreatment inside.
Prisoner Ombudsman for Northern Ireland Pauline McCabe, a colleague of Mr Masefield for the past five years, has commented on his stepping down.
She said: "I have worked with Robin Masefield over many years, and I am well aware of the incredibly high level of personal commitment he has brought to this challenging post.
"He has done so in what was, and continues to be, a very challenging time for the prison service.
"He has always taken a lead in working very helpfully and constructively with the Office of the Prisoner Ombudsman and this has been greatly valued and appreciated.