Beat 8 ounces room temperature cream cheese in a large bowl with a hand mixer until smooth.
Add 1 (3.4-ounce) box instant cheesecake pudding mix and beat again until fully combined and smooth, scraping down the sides and bottom of the bowl as needed.
Add 1 (8-ounce) container thawed whipped topping to the cream cheese mixture and beat until combined.
Stir in 1 (21-ounce) can cherry pie filling to the cream cheese mixture, followed by 3 cups mini marshmallows.
Cover the bowl with plastic wrap and chill for at least 1 hour before serving.
Notes
Leftover & Make Ahead Instructions
Fridge: Store leftover cherry cheesecake fluff in an airtight container in the refrigerator for up to 2 days.
Freezer: N/A—fluffs aren’t for freezing!
Make Ahead: The fluff requires at least 1 hour of chilling, but you can prepare it up to 4 hours ahead of serving. It is best served the same day it is made, so I would avoid preparing further than 4 hours in advance.