Abstract: A resilient ring-shape rod seal is to be installed in a rod seal groove in a cylinder in position to seal against a rod as it moves longitudinally with respect to the cylinder. The inside diameter of the ring-shape rod seal is the same as the nominal outside diameter of the rod. The rod seal installation tool of the invention includes three mutually parallel pins, pivotally mounted with respect to each other and designed to fit down over the ring-shape rod seal to have two outside pins situated inside of the rod seal ring and a center pin situated outside of the rod seal ring. Means are provided to pivot the outside pins out around the center pin and then toward each other to fold the rod seal in a generally C-shape configuration whereby the rod seal and tool can be inserted into the cylinder to align the rod seal with the cylinder rod seal groove. The pins are released to allow the seal to expand toward its nominal ring shape with at least part of the seal in the groove.