Combine brown sugar, corn syrup, and butter in a large saucepan; cook and stir over medium heat until mixture comes to a rolling boil.
Remove saucepan from heat; stir in peanut butter until mixture is smooth.
Place popcorn in a large bowl.
Pour peanut butter mixture over popcorn; toss until evenly coated. Form into about 16 balls using buttered hands. Arrange balls on a waxed paper-lined baking sheet; allow to dry and set, at least 30 minutes.
