Place marshmallows in a saucepan over medium heat; cook and stir until marshmallows are melted, 3 to 5 minutes.
Stir cream cheese, sour cream, pumpkin, and peanut butter together in a bowl. Beat marshmallows into the cream cheese mixture using an electric hand mixer until creamy. Chill in refrigerator 1 hour to allow flavors to blend.