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.

The latest audit of Leicester City Council's services shows 206 of 229 child protection plans were still inadequate or required improvement.
An Ofsted report last year said hundreds of children were facing delays in being allocated a social worker.
The authority said things were improving but changes would take time.
Updates from this story and more from Leicestershire
A judge in Leicester's Family Court recently criticised the city's children services after it emerged a nine-year-old boy had spent two years living with a convicted paedophile.
A city spokesman said the situation had improved since that case which dated back to a period of serious failures.
Councillor Lynn Moore, who chairs the city's scrutiny committee, said: "I am hopeful things will continue to improve but it is not an easy thing to bring about and Leicester is not the only authority that has these kinds of difficulties."
Analysis: BBC Radio Leicester political reporter Tim Parker
I think things are improving ... the earlier Ofsted highlighted the high turnover of social workers and managers leading to unacceptable delays in allocating children's cases.
But there are "early signs" of the workforce beginning to stabilise.
At one point, 30 of 62 social workers resigned in one section, leading to serious delays in nearly 300 vulnerable children being allocated a social worker when they needed one.
These signs of more stability in the workforce can only be good news.
Assistant mayor Sarah Russell, who took over responsibility for children's services after the earlier Ofsted report, said: "There is a huge mountain to climb - and it will continue to look that way for a while.
"[It's] better than when I took over nine months ago - but it will take another 15 to 18 months to get it right."
She said the city was trying to recruit new staff but it was difficult due to a national shortage of qualified social workers.
"We didn't know our services well enough - we didn't know well enough what was happening to children in our city, but we do now."
A fifth UK jihadist fighting for Islamic State militants in Syria has been added to a UN sanctions list.
[NEXT_CONCEPT]
It may take almost two years to fix "inadequate" children's services in a city that has failed to protect some vulnerable children, a new report says.