A bracket for holding and locking tablet-type electronic devices of various sizes securely includes a bracket base, two horizontally clamping members, and a vertically clamping member. The front side of the bracket base includes two horizontal rail grooves, which are adjacent to the bottom side of the bracket base and to two corresponding lateral edges of the bracket base respectively, and a vertical rail groove, which is adjacent to the top side and the center of the bracket base. Each horizontally clamping member can be inserted horizontally into the corresponding horizontal rail groove to any depth desired and then fixed in position. The vertically clamping member can be inserted vertically into the vertical rail groove to any depth desired and then fixed in position. Each of the clamping members is adjustable in position to adapt to the size of the tablet-type electronic device to be supported on the bracket.