The Hugo star is fronting the United Nations Children's Fund's annual campaign, which encourages American kids to make a difference to the lives of youngsters in poorer countries.
The 15 year old is urging her fans to collect money for the organisation as they visit houses this Halloween, while they can also set up fundraising pages on Unicef's website.
Moretz says, "Halloween is such a fun time of year. I love dressing up and taking part in festivities. That's why I can't think of a better way to honour this annual tradition than by supporting Unicef, an organisation that has saved the lives of millions of children across the globe.
"Trick-or-Treat for Unicef gives kids the freedom to choose how they'd like to express their support through a variety of activities."