"A child dies every 6 seconds of hunger, which is a huge statistic for me," Aguilera told Oprah Winfrey, announcing her appointment on Winfrey's popular TV talk show.
"After having my own child, I just had to be a part of it and do something about it and help change that situation," she said.
Aguilera, 29, whose hit songs include "Beautiful" and "Genie in a Bottle", has a two-year-old son with her music executive husband Jordan Bratman.
Last year, she traveled to Guatemala to see WFP's programs for maternal and child health programs, and in January she took part in the "Hope for Haiti" celebrity telethon that raised more than $60 million for earthquake disaster relief.
Aguilera described her work with WFP as a life-changing experience and said she plans to go to Haiti soon: "I want to check on the situation there and help to deliver food. I want to visit orphanages and schools there and try to do my part in helping."
Josette Sheeran, executive director of WFP, said Aguilera had a passion and dedication to hungry children that "will make her a wonderful champion in the fight against hunger.