Abstract: Systems and methods for validating online activities through proof-of-work techniques are provided. In one example, a validating computing system receives a request for a proof-of-work instruction from a client device that has submitted an online activity request to an online server system. The validating computing system generates and transmits a proof-of-work instruction for solving a problem to the client device. The validating computing system further receives a response to the proof-of-work instruction from the client device. The validating computing system generates a validity decision based on whether the client device correctly solved the problem, and transmits, to the online server system, the validity decision for use in granting the online activity request to the online server system.