A spokesman for United Nations Secretary-General António Guterres said the Israeli leader's vow to annex the Jordan Valley would be a "serious violation of international law".
The particularly strong rebuke from the world body follows a chorus of criticism against Israeli Prime Minister Benjamin Netanyahu for his promise to annex the area if he wins re-election next week.
In a statement, spokesman Stephane Dujarric also said the pledge would be "devastating" to the potential for peace with the Palestinians.
The UN statement comes after Saudi Arabia, a regional power that has grown closer to Israel in recent years, condemned the move.
Russia's Foreign Ministry also reportedly blasted the step ahead of a meeting between Netanyahu and Russian President Vladimir Putin on Thursday.