The poultry industry in the USA produced about 8.8 million broilers (Gallus gallus domesticus) in 2014 and generated an estimated 12 million Mg of broiler litter, a mixture of bird excreta, feathers, and bedding material. This byproduct is commonly used as fertilizer for crops and forages because it contains several macro and micro nutrients. In the broiler-rearing houses, ammonia (NH3) volatilization from broiler litter impairs bird health, decreases the fertilizer value of litter, and negatively impacts the environment. Gypsum has been proposed as a litter amendment due to its hygroscopic nature, but reports of NH3 abatement vary, and the mechanism responsible for NH3 reduction is not well understood. Laboratory studies were conducted to evaluate the effect of adding 20 or 40% flue-gas desulfurization gypsum (FGG) to broiler litter on litter water content, urea-degrading bacteria (UDB) and nitrogen (N) mineralization in litter, and to identify the mechanism responsible for reductions in NH3 loss. Results show that FGG can absorb moisture from litter, thereby increasing matric and osmotic stress, which led to a 38 to 71 % decrease in UDB. The stress encountered by microorganisms led to a 9.9 to 10.6 % increase in N mineralization possibly due to an increase in urease activity that ranged from 27 to 41 %. Amending litter with FGG also decreased litter pH by 0.09 to 0.84 pH units, with a consequent 18 to 28% decrease in NH3 volatilization. Experiments were conducted to better understand the mechanism responsible for pH suppression showed that the addition of gypsum to litter decreased pH immediately due to the precipitation of calcium carbonate (CaCO3) from gypsum-derived calcium and litter bicarbonate. As urea was hydrolyzed in gypsum-amended litter, additional CaCO3 precipitated and buffered against large increases in pH that accompany urea hydrolysis.