Mix marionberries, 3/4 cup sugar, flour, and lemon juice together in a large bowl until well-combined.
Fit the bottom of a 9-inch pie plate with 1 pie crust. Pour in marionberry mixture. Cover with second crust; crimp edges to seal. Brush pie lightly with water. Sprinkle 2 teaspoons sugar on top. Cut four to eight 1-inch slits in the top crust with a knife.
Bake in the preheated oven for 10 minutes. Reduce heat to 350 degrees F (175 degrees C) and continue baking until crust is golden, 45 to 50 minutes more. Let cool completely before serving, about 1 hour.