This paper describes the first measurement of fish passage in the Mekong River at Khone Falls. The site was in the Sadam Channel, which was modified in 2013 to improve fish passage and mitigate closure of Sahong Channel for the Don Sahong hydropower project. Underwater cameras recorded 149 hr of discontinuous video from January 18 to 26, 2015, which showed a major upstream migration by small cyprinid fish. Daily catch surveys in the same channel showed most fish migrated on days when video records were almost complete. We used stratified hourly sampling to review 17% of the available video and counted 14,783 fish and identified 16 taxa. The most abundant species were Labiobarbus leptocheilus, Henicorhynchus lobatus, and Henicorhynchus siamensis, and these fishes migrated almost exclusively during daylight. We calculated passage rates for West Sadam Channel from video samples and extrapolated those results to Sadam Channel, by assuming equivalent passage rates for both East and West Sadam Channels. This assumption was based on observations that fish were evenly distributed between both banks below the confluence, and they migrated close to each bank, so we assumed that there was an even split at the confluence and neither channel was preferred for upstream passage. Although channel modification improved fish passage efficiency, we estimated that artisanal fishers caught 79% of migrating fish in Sadam Channel, so fishing pressure remains the greatest risk to successful fish passage. Active fisheries management will be necessary to sustain and further improve passage efficiency in future.