Matt Baggott, Chief Constable of the Police Service of Northern Ireland, has announced he is to retire.
He will not seek a renewal of his contract with the PSNI when it ends in September.
Mr Baggott, who has been a police officer for 37 years, took up the job as head of the PSNI in August 2009.
Chief Constable Matt Baggott said: "It has been a real privilege to lead the PSNI through the immense changes of the past few years. I am deeply proud of the courage and commitment of my colleagues and the enormous progress they have made on behalf of everyone.
"In my remaining months as Chief my priorities will be to ensure the PSNI has the resources to deal effectively with the many challenges ahead and that our very personal, professional and protective service goes from strength to strength."
The announcement could leave a vacuum with no chief constable or deputy chief constable of the PSNI. Deputy Chief Constable Judith Gillespie announced just last month that she would be retiring in March.
Mr Baggott's last year has been swamped with political rows, flag protests and the threat from dissident republicans.
His tenure has seen him deal with annual riots at Ardoyne in north Belfast on July 12 over a contentious loyal order parade, and with the severe threat of dissident attacks on colleagues. He announced the death of new recruit Constable Ronan Kerr in 2011 after a car bomb blew up outside his house.
Last year his force was targeted to ferocious attacks by loyalists angry at the decision to restrict the flying of the Union flag from Belfast City Hall. His officers have been attacked using ceremonial swords, scaffolding poles and bricks.
Mr Baggott was vocal in urging a political solution to sectarian tensions in Northern Ireland as his force came under fire and called for extra funds to employ more officers.
More broadly, he has pointed to a drop in recorded crime levels and increased engagement with police across the community following reforms as Northern Ireland emerged from conflict.
Reacting to the news of the Chief Constable's retirement, DUP group leader on the Northern Ireland Policing Board, Jonathan Craig MLA acknowledged Mr Baggott had served during challenging times.
He said: "He carried out his duties with sincerity and warmth and whilst we did not always agree with some of the decisions he made, I and my colleagues always found him willing to listen and take our views on board.
"The Chief Constable's job is one of the most difficult in Northern Ireland and Mr Baggott tried his best to serve our community and make our society a safer, better place. We wish him well in his retirement."
Northern Ireland Policing Board chairwoman Anne Connolly said Mr Baggott's leadership was appreciated: "The Chief Constable of the PSNI is undoubtedly one of the most demanding jobs in policing and during his tenure Matt has dealt with some of the most challenging issues in ensuring the safety and security of our community.
"A strong advocate for community policing, Matt has worked to make sure his officers and staff deliver the best possible service to the community and build on the considerable policing change programme here."
The policing board will discuss the appointment process for replacing the Chief Constable and Deputy Chief Constable at its February meeting.
If he stays until his contract ends in September, Mr Baggott will face one last marching season.
The married father-of three, who is originally from south London, was previously the head of the Leicestershire Constabulary when he took over from Sir Hugh Orde.
He was the firm favourite because he was seen as a strong advocate of community-style policing, and he had worked with former RUC chief constable Sir Ronnie Flanagan on the National Review of Policing in the UK.
He was also a leading team member of the Stephen Lawrence inquiry team.
His appointment was historic because it was the first time Sinn Fein has been involved in the selection of a police chief for Northern Ireland. MLA Alex Maskey sat on the interview panel - and the decision was unanimous.
When he was appointed Northern Ireland’s new Chief Constable, Matt Baggott was described as a safe pair of hands for the PSNI.
The former Leicestershire Constabulary chief wass a devout Christian committed to the community-style policing favoured by the Northern Ireland Policing Board.
He spent the first 20 years of his service in the Metropolitan Police leading policy reviews on issues such as partnership, regeneration and inner city crime.
For 18 months he was staff officer to the then commissioner, Sir Paul Condon, and headed the Metropolitan Police team assisting the Stephen Lawrence Public Inquiry.
In 1998 he became assistant chief constable in the West Midlands with specific responsibility for policing diversity, crime and disorder, professional standards and criminal justice.
He joined Leicestershire Constabulary on promotion to Chief Constable in December 2002.
Mr Baggot was Vice President of the Association of Chief Police Officers 2004-07, has been a member of the National Policing Board and has advised the Government on a range of issues from partnership through to social cohesion.
He has also worked with former PSNI Chief Constable Sir Ronnie Flanagan on the National Review of Policing.
During his time in Leicester he worked closely with the large Muslim community and has been praised for publishing his expenses online.
For the most part he managed to avoid controversy, but in 2006 was forced to apologise for his officers’ behaviour when a whistleblower revealed to Channel 4’s Dispatches team that officers had watched porn films while on duty, played hide and seek in their cars and fetched takeaways while pretending to be busy.
At the time he said: “I apologise for the poor behaviour shown in the documentary. And we will accept criticism wherever it is justified.”
Mr Baggott was awarded a CBE in the 2008 New Years Honours, received the Queen's Police Medal in June 2004, was elected a Fellow of University College London in 2006, and awarded an Honorary Doctorate of Letters by De Montfort University, Leicester, in July 2007.
Commentators in Leicestershire expressed surprise when he threw his hat in the ring for the PSNI job.
It was thought he had hoped to return to a top position within the Metropolitan police.
Additional reporting by PA