Press the pie crust into the bottom and up the sides of a removable-bottom tart pan. Prick the bottom of the crust all over with a fork.
Bake for 10 to 12 minutes, until golden brown. Remove from the oven, reduce the oven temperature to 375 degrees and set aside.
Melt the sweetened condensed milk and chocolate chips together in a medium saucepan over medium heat. When the mixture is smooth, pour it into the prepared tart crust and smooth the surface. Top the filling with the marshmallows and spread out in an even layer.
Return the tart to the oven and bake for 3 to 4 minutes, until the marshmallows are golden.