Detecting biosignature gases on exoplanet atmosphere with near-future space telescopes is one of the most promising methods of detecting life beyond Earth. However, only a handful of biosignature gases are discussed today, and some can also be made by non-living, geological processes. Life, however, produces thousands of gases for a wide variety of purposes. Here we present isoprene, C5H8, as a potential biosignature gas. On Earth, isoprene is made at a comparable rate to methane (~500 million tonnes per year) and solely by living organisms. Remarkably, isoprene is produced by many organisms; plants, bacteria, and animals. Unfortunately, isoprene is rapidly destroyed on Earth by oxygen and OH, so for modern Earth isoprene is a poor biosignature, but on a world without oxygen, could this abundant gas be a sign of life? We evaluated the observation time required to detect isoprene in various anoxic atmospheres and found that detection is possible using JWST if life on that world made only one third as much isoprene as Earth life does. Despite the observational challenges, isoprene should be considered as a potential biosignature gas because of wide and abundant production by life on Earth and no false positives in any planetary scenario.