The Southern Distinct Population Segment of Green Sturgeon Acipenser medirostris spawns in the Sacramento River, California, and is listed as a threatened species under the U.S. Endangered Species Act. We estimated the spawning run size and population size in 2010–2015 by using dual-frequency identification sonar (DIDSON) sampling, underwater video camera species identification, and acoustic tag detections. Spawning run size varied from 336 to 1,236 individuals. We estimated the total population size to be 17,548 individuals (95% confidence interval [CI] = 12,614–22,482). The estimated number of adults was 2,106 (95% CI = 1,246–2,966), the estimated number of juveniles was 4,387 (95% CI = 2,595–6,179), and the estimated number of subadults was 11,055 (95% CI = 6,540–15,571). This study provides the first estimate of Sacramento River Green Sturgeon run size and initiates a time series of abundance that can inform Endangered Species Act recovery processes. Furthermore, these absolute abundance estimates provide a context for evaluating the significance of impacts, such as bycatch in coastal fisheries or entrainment in water diversions, where the number of impacted individuals is known.