The Dhoom 3 star will support the organisation's work in promoting children's rights and nutrition, as half of all children in India are malnourished.
Khan said: "I am honoured to formalise my association with UNICEF as their Ambassador to help raise awareness on the issue of malnutrition amongst children.
"I realise there is a lot to be done and I hope that by lending my voice I can make a difference to the lives of children and hereby to the future of our country. Partnering with UNICEF will enable me to advocate for the rights of the most vulnerable and marginalised children in India."
UNICEF representative Karin Hulshof welcomed the appointment.
"We know Aamir Khan as a leading star of Indian cinema," she said. "He is also a compelling advocate, with an enormous drive and commitment, to transform and improve the situation of children in the country."