A leftover burger can become another meal, but good storage matters more than finding a clever reheating trick. Start by deciding whether the food has been handled safely enough to keep.
Refrigerate promptly
USDA advises refrigerating perishable leftovers within two hours, or within one hour when the surrounding temperature is above 90°F. Keep the refrigerator at 40°F or below. If food has sat out too long, do not rely on reheating, smell or appearance to make it safe.
Refrigerated leftovers are generally used within three to four days under USDA guidance. Freeze promptly if you will not use them in that window, and label the container so the timing is clear.
Separate the parts
When practical, store the cooked patty separately from the bun and cold toppings. This lets you reheat the meat without warming the lettuce and tomato at the same time. It also keeps sauce and moisture from soaking the bread during storage.
Reheat the meat thoroughly
USDA recommends reheating leftovers to 165°F, checked with a food thermometer. Choose a method that heats the center evenly. If using a microwave, follow suitable covering and standing-time guidance and check for cold spots.
Toast the bun separately if desired, then assemble with properly stored toppings. Texture may differ from a freshly cooked burger, and that is normal.
For cooking a new burger rather than reheating one, use the burger temperature guide. Food-safety targets and quality preferences should be considered separately.