161989 Cacus (provisional designation: ) is a stony asteroid, classified as near-Earth object and a potentially hazardous asteroid of the Apollo group, approximately 1 kilometer in diameter. It was discovered on 8 February 1978, by German astronomer Hans-Emil Schuster at ESO's La Silla Observatory in northern Chile. Its orbit is confined between Venus and Mars.
This minor planet was named from Roman mythology, after Cacus, a fire-breathing monster, which was killed by Hercules. The official naming citation was published by the Minor Planet Center on 24 November 2007 ().