UN High Commissioner for Human Rights Zeid Raad al-Hussein cited Mr Trump's comments on the use of torture and his attitude to "vulnerable communities".
The Republican presidential candidate's campaign has been marked by a number of controversial comments.
Recent crude remarks about women have caused the biggest political fallout.
Mr Hussein said at a news briefing in Geneva: "If Donald Trump is elected on the basis of what he has said already - and unless that changes - I think it is without any doubt that he would be dangerous from an international point of view."
Mr Hussein said he was "not keen or intent on interfering in any political campaign within any particular country".
But he said that when an election could result in an increase in the use of torture "or the focus on vulnerable communities in a way that suggests that they may well be deprived of their human rights, then I think it is incumbent to say so".