The World Health Organization has said it will no longer recruit people who smoke or otherwise use tobacco.
Smoking is a leading cause of chronic disease
The ban, which came into effect on Thursday, applies to any applicants who smoke and say they would continue to smoke, either daily or occasionally.
Tobacco chewers and those who use snuff tobacco are also affected.
The WHO said the decision was based on its position at the helm of the global campaign against smoking. Existing staff will be encouraged to quit.
The UN agency, which is based in Geneva, Switzerland, employs some 2,400 people.
In a letter distributed to staff, the WHO said it needed to ensure that its efforts to curb the "tobacco epidemic" were reflected internally.
Advertisements for positions at the WHO now carry the notice: "WHO has a smoke-free environment and does not recruit smokers or other tobacco users."
Spokesman Iain Simpson told the AFP news agency: "The WHO is campaigning against tobacco and the tobacco industry, and has led efforts to adopt the international anti-tobacco treaty. For us it's a question of principle." The decision to ban smoking came one day after several hundred staff staged a temporary walkout in protest at job cuts.