Mix popcorn, peanuts, and pretzels in a large bowl. (If you don't have a giant bowl, use two medium bowls)

Melt butter and sugar in a microwave safe bowl for 60 seconds. Stir.

Add marshmallows and microwave in 30-second intervals for 90 seconds, stirring after each interval.

Stir and pour over popcorn mixture. Mix thoroughly.

You can dig right in if you want the warm ooey-gooey popcorn, or wait about 10 minutes for it to slightly set up.