Grease a 9x13-inch baking dish with butter or non-stick spray to prevent sticking.
Pour the marshmallows and butter into a large saucepan and heat over low to medium heat, stirring constantly until the marshmallows are completely melted and the mixture is smooth.
Add the pumpkin purée, cinnamon, and nutmeg to the melted marshmallow mixture, stirring until well combined and the mixture turns a vibrant orange color.
Remove the saucepan from heat and immediately pour the rice cereal into the pumpkin-marshmallow mixture. Gently fold with a spatula to evenly coat all the cereal without crushing it.
Quickly transfer the mixture into the prepared dish, pressing down gently with a spatula or your hands to spread it into an even layer and compact it slightly.
Allow the treats to cool completely at room temperature for about 20-30 minutes, until they are firm and set.
Once cooled, cut the pumpkin Rice Krispie treats into squares or rectangles of your desired size.
Serve and enjoy their vibrant orange color and chewy-crunchy texture, perfect for fall festivities or Halloween parties.