An adjustable projector bracket assembly has a pivoting device, a moving device and an adjusting device. The moving device is slidably connected with the pivoting device and has a bottom. The adjusting device is mounted on the bottom of the moving device and has a projector bracket and at least three supporting arms. The projector bracket has at least three arm grooves formed through the projector bracket. The at least three supporting arms are respectively pivotally and slidably connected with the at least three arm grooves. With the supporting arms pivotally and slidably connected with the arm grooves, projectors P of different sizes or having different assembling holes can be mounted securely on the projector bracket.