Abstract: A knot-tying assembly is provided for tying overhand knots in an elastic material, such as the stem of an inflated balloon. The knot-tying assembly includes a mounting base for attaching the knot-tying assembly to a structure, such as an inflation gas regulator, and support arm rigidly connected to and extending outward from the base. The support arm includes an extension at the end distal from the base with a recessed mounting surface. A pair of posts is mounted on this recessed mounting surface with the posts positioned substantially parallel to each other and spaced apart a distance that defines a ring-retaining slot. During operation, the stem of an inflated balloon is positioned within the slot, and the ring is retained in position, at least in part, by the two posts. The knot-tying assembly further includes a slidable, hollow form which is positioned in contact with the outer surface of the support arm.