A self-lcoking bill accumulator (10) having tamper-proof features is provided for use with a dollar bill processing machine. During use, the bill accumulator (10) is mounted to a bill processing machine. The accumulator has a double locking feature whereby rotation of an outer lock (48) by means of a key (52) is required in order to detach and remove the bill accumulator (10) from the dollar bill processing machine. This rotation of the outer lock (48) occurs prior to removal of the bill accumulator and causes a corresponding rotation of an inner lock (24, 44), which in turn positively locks the bill accumulator (10) to protect the bills (32) in the accumulator from pilferage after removal. Upon removal of the bill accumulator (10), the outer lock (48) becomes disengaged from the inner lock (24, 44) such that a separate key is required in order to "unlock" the bill accumulator (10) and remove the bills (32).