The agency also said a spike in Israeli settlement building and confiscations of Palestinian land, water and other resources were keeping poverty and joblessness at intolerable levels.
In a report, the agency said international donor support for Palestinian territories fell by 38 percent between 2014 and 2016, "due in part to the fact that occupation (has prevented) previous aid flows from translating into tangible development gains."
Israel has fought three devastating wars since 2008 against the Palestinian Islamist Hamas movement in charge of the Gaza strip.
Israel has maintained a decade-long blockade on the coastal enclave which it says is necessary to prevent arms and materials reaching Hamas militants.
UN chief Antonio Guterres visited Gaza in July and called the blockade "one of the most dramatic humanitarian crises" he had seen.
In its annual report two years ago, the UN agency warned that the situation was so dire that the Gaza Strip could become uninhabitable by 2020 without drastic action to rebuild the area ravaged by wars.
But rather than improvements, "the humanitarian and economic situation has instead worsened since then", Tuesday's report said.
Denouncing a "protracted, unprecedented level of human suffering in Gaza", it said that 80 percent of Gaza's population currently receives humanitarian aid, while half of inhabitants do not know where their next meal is coming from.
Settlement boom
Palestinians have for the past 50 years of occupation faced "suppressed human potential, ... (leaving the) economy incapable of employing one-third of its workforce," the report said.
Lacking access to land and water, as well as an import ban on suitable fertilisers and other restrictions, for instance helped shrink agricultural output in the West Bank by 11 percent between 2015 and 2016.
"The fact that today, real GDP per capita in the (West Bank) is at the same level as in 1999 is a clear indication of the human cost and lost economic potential resulting from the occupation," the report said.
The per capita gross domestic product in Gaza, meanwhile, had shrunk by nearly a quarter since 1994.
Unemployment, especially among young people, was particularly concerning, the report said.
Despite international demands for Israel to halt its settlement activity, Tuesday's report said that the country had sped up development this year, following a 40 percent hike in 2016 over 2015.
"In recent years, population growth in settlements has not only surpassed population growth in Israel but also that of the Palestinian population," it said.
Since 1995, the settler population has more than doubled to between 600,000 and 750,000 people, it said.
The overall jobless rate stood at 27 percent in the West Bank and at 42 percent in Gaza last year.
For young people aged 15 to 29, the official unemployment rate is 27 percent in the West Bank and 56 percent in Gaza -- the highest rate in the region.
And even those figures do not tell the full story, since they do not include the nearly 60 percent of Palestinian youths who have given up on seeking work or attending school.
