Mrs Iro began her career in health as a nurse before going to become the Cook Islands senior health official and the first woman to hold the post.
In making the appointment, the WHO Director-General, Dr Tedros Adhanom Ghebreyesus said nurses are central to achieving universal health coverage and the Sustainable Development Goals.
He said Ms Iro will keep that perspective front and centre at the WHO.
The appointment fulfils a commitment Dr Ghebreyesus made when he became director-general to appoint a nurse to his senior team.
Photo: Flo Syme Buchanan Liz Iro, World Health Organisation Chief Nursing Officer