At least 60 prisoners have been executed since the start of 2014; with a total of 1,724 people on the death row, according to the UN.
The UN voiced alarm over the increasing use of the death penalty in Iraq in the decade since its reintroduction, warning such punishments would only fuel deadly sectarian conflict.
In a report released on Sunday, the United Nations office in Iraq said the increase culminated with the execution of 177 individuals in 2013, including up to 34 in a single day.
It also said at least 60 had been executed since the start of 2014 and that a total of 1,724 prisoners were on death row as of August.
"The large numbers of people who are sentenced to death in Iraq is alarming, especially since many of these convictions are based on questionable evidence and systemic failures in the administration of justice," said the UN's envoy in Iraq, Nickolay Mladenov.
The UN office said half the trials it monitored saw judges systematically ignore claims by defendants that they were subjected to torture to induce confessions.
It also said most defendants appeared in court unrepresented. When any lawyer was appointed, the defence was not granted enough time to prepare its defence.
"Far from providing justice to the victims of acts of violence and terrorism and their families, miscarriages of justice merely compound the effects of the crime by potentially claiming the life of another innocent person," the report said.
The UN High Commissioner for Human Rights, Zeid Ra'ad Al Hussein, warned that such court decisions would only fuel the flames of deadly sectarian conflict in Iraq.
"Given the weaknesses of the criminal justice system in Iraq, executing individuals whose guilt may be questionable merely compounds the sense of injustice and alienation among certain sectors of the population," he said.