Preheat your oven to 425°F (220°C).
Prepare the pie crust according to package instructions or your homemade recipe. Roll out one crust and fit it into the bottom of a 9-inch pie pan. Trim the edges as needed.
In a mixing bowl, combine the chopped rhubarb, granulated sugar, flour, ground cinnamon, and lemon juice. Mix well to coat the rhubarb evenly in the dry ingredients.
Pour the rhubarb mixture into the prepared pie crust, spreading it evenly. Dot the mixture with small pieces of butter.
Roll out the second pie crust and place it over the filling. You may create a lattice pattern or simply cover it entirely. Trim and crimp the edges to seal the pie. Cut a few slits in the top crust to allow steam to escape.
Place the pie on a baking sheet to catch any drips. Bake in the preheated oven for 15 minutes.
Reduce the oven temperature to 350°F (175°C) and continue baking for an additional 35–40 minutes, or until the crust is golden brown and the filling is bubbly.
Remove the pie from the oven and let it cool on a wire rack for at least 2 hours before slicing to allow the filling to set.