Pre-heat the oven to 175 degrees Celsius (350 degrees F) and gather all ingredients.
Mix well condensed milk with key lime juice, sour cream and lime zest. Pour into the graham cracker pie crust. Place on a baking tray.
Bake the pie in a preheated oven between 5 and 8 minutes until you see tiny bubbles on the top surface. The filling should be a little wobbly, but not brown.
After cooling the pie, chill it in the fridge for about an hour before serving. If desired, garnish with lime slices or whipped cream.
Enjoy and serve!