According to Fox, Allen will travel to Haiti with the United Nations Foundation to highlight the country's continuing need after last month's disastrous earthquake.
The singer will perform on the February 25 Idol results show and share live footage from his trip.
Viewers will be able to donate online and via text message to the UN's relief effort in Haiti.
"It is heartbreaking to see the devastation that Haiti is currently facing. I am truly honoured to be travelling to Haiti to see the UN's relief efforts firsthand and to help out any way that I can," Allen said in a statement.
Fox will also air a two-hour Idol Gives Back special on April 21 to benefit Save the Children, the Children's Health Fund, Feeding America, Malaria No More and the United Nations Foundation.