The 10-meter South Pole Telescope (SPT) recently started an untriggered millimeter-wave transient search, revealing several exciting candidates for high-energy transient signals such as gamma ray burst (GRB) afterglows and flaring variable stars. Though it has long been known that astrophysical transients are detectable at millimeter wavelengths, the millimeter-band time-variable sky is largely unexplored. Follow-up observations of transient signals discovered at other wavelengths and a previous pathfinder survey with the SPT show the potential for discovery of high-energy astrophysical phenomena in this band. Since 2018, the third generation SPT camera (SPT-3G) has been making daily observations of a 1500 deg2 patch of the Southern sky in three bands centered at 95GHz, 150GHz and 220GHz. Though primarily operating as a Cosmic Microwave Background observatory, SPT-3G’s arcminute resolution and instrumental noise levels give it a combined sensitivity to day-scale transients of 30-40 mJy, scaling with the inverse square root of time to sub-10 mJy sensitivity for week-scale and longer flares, thus enabling the detection of a large variety of transient signals at different timescales. Here I will review the SPT-3G transient search program and present first results from the 2019-2020 observing seasons.