Cold pizza reheated the wrong way turns either rock-hard or soggy in the middle, both of which are avoidable once you understand what the microwave is actually doing to the crust.
Microwaves add moisture through steam, which is exactly what a crispy crust doesn't need. The result is a slice that's hot but limp, with a crust that's gone rubbery instead of staying crisp.
Heat a dry skillet over medium heat. Add the pizza slice and cook for 2-3 minutes, until the bottom of the crust is crisp again. Then add a few drops of water to the pan around the slice (not on top of it) and cover with a lid for another minute or two. The trapped steam melts the cheese on top while the direct heat keeps the crust crisp underneath, giving you the best of both textures.
Preheat your oven to 375°F, place the slices directly on the oven rack or a baking sheet, and heat for 8-10 minutes until the cheese is melted and the crust has crisped back up. Placing slices directly on the rack (with a sheet below to catch drips) gives the crispest result, since air circulates underneath the crust.
An air fryer at 350°F for 3-4 minutes gets a similar result to the skillet method with less hands-on attention, and works especially well if you're only reheating one or two slices.
Pizza keeps for 3-4 days in the fridge. It reheats well multiple times using the skillet, oven, or air fryer method, since none of those methods depend on how fresh the slice was to begin with, just on avoiding the microwave.
For general reheating rules that apply across other meal-prep staples, see the complete meal-prep reheating guide, or check out our guide on reheating pasta for another Italian-leaning leftover that benefits from the same skillet-and-cover approach.