Denis J. Halliday (born c.1941) is an Irish diplomat. He was the United Nations Humanitarian Coordinator in Iraq from 1 September 1997 until 1998, and earlier the Deputy Resident Representative to Singapore of the United Nations Development Programme.

Graduated from Trinity College, Dublin, with a MA in Economics, Geography and Public Administration; Halliday spent a year in Kenya as a Quaker volunteer from 1962 to 1963, before joining United Nations in 1964. His first posting was in Tehran as a junior professional officer in the United Nations Technical Assistance Board and Special Fund. From 1966 to 1972, he served in the Asian Bureau of the UNDP Headquarters in New York and then in Malaysia from 1972 to 1977 as Deputy Regional Representative. Moving to Indonesia, he continued as Deputy Regional Representative until 1979, when he moved to Samoa as Resident Representative covering Samoa, the Cook Islands, the Tokelau Islands and Niue in the South Pacific. Returning to New York in 1981, he served in the Asia and Pacific Bureau and in 1985 became the Deputy Director, Division of Personnel and Chef de Cabinet in 1987. In 1985, he took up the post of Deputy Director, Division of Personnel before becoming Chef de Cabinet, Office of the Administrator of the UNDP from 1987 to 1989. He served as the Director, Division of Personnel of the UNDP from 1989 to 1994. He then served briefly as UNDP Regional Representative in Thailand, before taking up the post of Assistant Secretary-General for Human Resources Management of the United Nations, based in its New York Headquarters, serving from 1994 to 1997, when he was appointed United Nations Humanitarian Coordinator in Iraq.

After a 34-year career at the United Nations, where he had reached Assistant Secretary-General level, Halliday resigned in 1998, over the sanctions against Iraq, characterizing them as "genocide". He subsequently gave the following explanation of his decision to resign:

I often have to explain why I resigned from the United Nations after a 30-year career, why I took on the all-powerful states of the UN Security Council; and why after five years I continue to serve the well being of the people of Iraq. In reality, there was no choice, and there remains no choice. You all would have done the same had you been occupying my seat as head of the UN Humanitarian Program in Iraq.

I was driven to resignation because I refused to continue to take Security Council orders, the same Security Council that had imposed and sustained genocidal sanctions on the innocent of Iraq. I did not want to be complicit. I wanted to be free to speak out publicly about this crime.

Above all, my innate sense of justice was and still is outraged by the violence that UN sanctions have brought upon, and continue to bring upon, the lives of children, families – the extended families, and the loved ones of Iraq. There is no justification for killing the young people of Iraq, not the aged, not the sick, not the rich, not the poor.

Some will tell you that the leadership is punishing the Iraqi people. That is not my perception, or experience from living in Baghdad. And were that to be the case – how can that possibly justify further punishment, in fact collective punishment, by the United Nations? I don't think so. And international law has no provision for the disproportionate and murderous consequences of the ongoing UN embargo – for well over 12 long years.

In 2003 Halliday was presented with the Gandhi International Peace Award in recognition of his work drawing attention to the plight of Iraqis. In 2007, he presented the same award to the website Media Lens, whose co-founder David Edwards had interviewed Halliday in May 2000 about his work in Iraq. In 2009 Denis Halliday agreed to become a Patron of the Gandhi Foundation, and he presented the annual peace award to the Children's Legal Centre.