Preheat oven to 325 degrees F. Meanwhile, stir together the melted butter, brown sugar, salt and pumpkin spice. Pour over the popcorn and stir until even distributed and coated.
Dump into a 13x9 pan and bake for 10 minutes. Stir and return to the oven for 5 more minutes. Let cool in the pan and enjoy. Store in an airtight container.