Preheat the oven to 400°F (200°C).
In a large bowl, combine Saskatoon berries, sugar, cornstarch, and lemon juice. Toss until the berries are well coated.
Line a pie dish with one half of the pie crust. Pour the berry mixture into the crust, and dot with small pieces of butter.
Cover with the top crust, trimming any excess and crimping the edges to seal. Cut slits in the top to allow steam to escape.
Bake in the preheated oven for 45 minutes, or until the crust is golden brown and the filling is bubbling.
Allow to cool before serving.