Pop popcorn and place in a large bowl, removing any un-popped kernels.
Melt butter in a medium saucepan over medium-low heat.
Add brown sugar and corn syrup and cook, stirring, until well blended.
Add marshmallows and stir continuously until all melted, about 5 minutes.
Pour mixture over popcorn and mix well. Spread popcorn out on parchment paper or a greased baking sheet and let sit for about 10 minutes to set up.