A method of injection molding an opening device on sheet packaging material for packaging pourable food products; the packaging material has at least a first and a second layer, and an opening formed through the first layer and sealed by a cover portion of the second layer; the method includes the steps of injecting molten plastic material onto a first side of the cover portion of the second layer, feeding the molten plastic material along the cover portion to cover it on the first side, and feeding the molten plastic material through an annular peripheral portion of the cover portion to form a spout of the opening device projecting from a second side, opposite the first side, of the cover portion; the molten plastic material covers the cover portion to define, with the cover portion, a user-removable sealing portion of the spout.