Aseel Muthana, from Cardiff, followed his older brother Nasser - who is already on the UN's list - in travelling to Syria in early 2014.
From Syria the 18-year-old told the BBC in an online interview last year: "I am willing to die but Allah knows the truth behind the words."
He faces a travel ban and a freeze of his assets.
The other four UK jihadists fighting or recruiting for Islamic State militants in Syria who are subject to UN sanctions at the UK's request are: Aseel's brother, Nasser Muthana, from Cardiff, Omar Hussain from High Wycombe, Aqsa Mahmood from Glasgow and Sally-Anne Jones from Chatham in Kent.
The move is a new tactic to stem the flow of recruits to the IS group.
At least 700 Britons have travelled to support or fight for jihadist groups in Syria and Iraq, according to British police. About half have since returned to the UK.
The UN sanctions regime was set up to tackle suspected al-Qaeda terrorism and extended to IS.
The UN list contains the names of 72 organisations and 231 individuals. British nationals Abu Hamza al-Masri, currently serving a life sentence in prison after being found guilty of supporting terrorism, and Syrian-born Mohammed al Ghabra, 35, from east London, who has been linked to al-Qaeda, were already on the list.
A fifth UK jihadist fighting for Islamic State militants in Syria has been added to a UN sanctions list.