The actor has been an ambassador for the children's charity for several years, and is requesting for donations to help children in east Africa suffering from drought.
In the promo, he reveals that £20 million will need to be raised to bring supplies into the area.
"In just one refugee camp in Kenya, there are nearly half a million people in urgent need of food, water and basic healthcare," he says.
"The situation throughout the region is becoming more and more critical; the threat of starvation is very real. £5 does not go far here in the UK, but for a child in east Africa it could be the difference between life and death."
McGregor and wife Eve Mavrakis revealed that they have adopted a new baby girl last month. They have previously adopted daughter Jamiyan from Mongolia in 2006.
Watch Ewan McGregor in a previous UNICEF appeal video below: