A novel docking seal was investigated for application to the main interface seal of NASA’s low-impact docking system. This interface seal was designed to maintain acceptable leak rates while being exposed to the harsh environmental conditions of outer space. In this experimental evaluation, the leak rate of a candidate docking-seal assembly was characterized and evaluated. The composite seal was manufactured from silicone elastomer S0383-70, vacuum molded into a metal retaining ring. Four seal designs were considered with unique characteristic heights. The leak-rate performance was characterized through a mass point leak-rate method by monitoring gas properties within an internal control volume. The leak rates of the seals were measured at representative docking temperatures of −58, +73, and +122°Ffor all four seal designs and characterized at 100, 74, and 48% of full closure. For all conditions, considered the candidate seal assemblies met the leak-rate criteria derived from the approximate mission requirements.