The device was discovered last week, southeast of Tucson, Arizona, when agents spotted a group of men scattering as they approached.
A closer look turned up two bundles of cannabis weighing a combined 47lb (21kg), which had yet to be launched.
The catapult was dismantled on the Mexican side. No arrests were made.
About 650 miles of the 1,100-mile (1,770km) border is covered by some sort of wall or fence.
Donald Trump has committed himself to building more walls, but drug traffickers have in recent years turned to increasingly creative means of getting their product over, including drones, car ramps, and air cannons.
Traffickers have also made extensive use of tunnels under the border. In March, authorities uncovered a 380m tunnel that ran from a restaurant in Mexico to a house in California.
US border patrol agents have found a medieval-style catapult mounted on the border wall with Mexico, designed to launch bundles of drugs into the US.