Abstract: An AC window security device is provided, comprising: a sill plate bracket, a cross-bar, a pair of U-brackets, and a pair of J-brackets. The sill plate bracket is configured to be secured to a window sill under a window AC unit in an at least partially open window. The lower end of each U-bracket hooks into an opening in one end of the sill plate bracket and extend vertically along a side of the AC unit. The upper end of each J-bracket fits over an upper frame of the at least partially open window. The upper end of each U-bracket is removably secured to the lower end of a corresponding J-bracket. The cross-bar is configured to be positioned along a top surface of the AC unit and removably secured to both U-brackets, wherein the at least partially open window is prevented from being opened.