In a large mixing bowl, combine the water, salt, and sugar. Stir until the salt and sugar are fully dissolved.
Add the garlic, peppercorns, rosemary, and apple cider vinegar (if using) to the brine mixture. Stir to combine.
Submerge the deer meat in the brine, ensuring it’s fully covered. You can place it in a large container or a ziplock bag for easy storage.
Refrigerate for at least 12 hours, up to 24 hours, depending on the thickness of the meat.
After brining, remove the meat from the brine, pat it dry with paper towels, and cook as desired.