The experts said imposing the death penalty on someone who was a child at the time of offending and after allegations of torture was "incompatible with Saudi Arabia's international obligations".
International pressure is mounting on Saudi Arabia to stay the execution of young Ali Mohammed al-Nimr, accused of taking part in anti-government protests.
The UN has issued an urgent call for the the Saudis to save the man who faces imminent beheading and crucifixion for crimes he reportedly committed as a child, reports The Independent.
A Saudi court has upheld the sentence of Mr al-Nimr, now 21, the son of a prominent government dissident, despite growing and high-level international condemnation.
Mr al-Nimr, who was arrested in 2012 for his participation in Arab Spring protests when he was just 17, could now be put to death at any time.
The young man's case has been the subject of fervent campaigning from rights groups including Amnesty International and Reprieve, who say he was tortured and forced to sign a false confession before being sentenced to "death by crucifixion".
Now, a group of UN human rights experts have penned a joint statement calling on Saudi Arabia to "immediately halt the scheduled execution" and give Mr al-Nimr "a fair retrial".
The French government has also taken the unusual step of adding its voice to calls for a stay of Mr al-Nimr's execution. Foreign ministry spokesman Romain Nadal said: "France is concerned about the situation of Ali Mohammed al-Nimr, who was sentenced to death even though he was a minor at the time of the events."