Ammonia is an important compound in freshwater ecosystems. It can stimulate phytoplankton growth, exhibit toxicity to aquatic biota, and exert an oxygen demand in surface waters. In productive lakes, ammonia commonly accumulates in bottom waters in conjunction with the onset of anoxic conditions, thereby degrading lake water quality. Sediment cores from 12 lakes and reservoirs were incubated in experimental chambers to examine how dissolved oxygen levels impact sediment release of ammonia. Sediments from all sites released ammonia under anoxic conditions. Release rates for oligo/mesotrophic, meso/eutrophic, and eutrophic/hypereutrophic sites were <5, 5–10, and >15 mg-N m-2 day-1, respectively. Under oxic conditions, ammonia release was negligible in sediments from oligotrophic and mesotrophic sites, and generally lower or reversed in sediments from eutrophic sites. Ammonia release under anoxic conditions is the result of ammonia build up in sediments due to a loss of biological nitrification and a decrease in ammonia assimilation by anaerobic microorganisms. Lake oxygenation, the dissolution of pure oxygen gas into surface waters, can improve water quality in the bottom of thermally stratified lakes, and potentially in shallow lakes, by eliminating sediment ammonia release through the maintenance of a well-oxygenated sediment–water interface.