Cut apple into quarters; remove core. Cut each quarter into 4 slices. Spread 1/2 teaspoon peanut butter on one side of each slice.
With kitchen scissors, cut each marshmallow in half lengthwise. Place 4 marshmallow halves over peanut butter on 8 apple slices, extending slightly over peel of slice, for teeth.
Top each with remaining apple slice, peanut butter side down, to make smiling mouth.