A new approach to digital watermarking in three-dimensional (3-D) space based on virtual-optics and random mask encoding is presented. The watermarking algorithm can offer an effective solution to the issue of copyright protection of multimedia data distributed via the communication channel.The results show that the embedded watermark is robust to normal data manipulations, such as adding noise, cropping, rotation, and JPEG compression. Because the design concept and methodology based on geometric parameters of Virtual-Optics was adopted, the proposed 3-D watermarking system has good security. There are large degrees of freedom existing for designing a security lock(s)/key(s), including multiple lock(s)/key(s) and multi-dimensional lock(s)/key(s). Numerical experiments are performed to validate the proposed method, and sensitivities of some parameters are quantitatively analyzed and illustrated. The possible dimensions of keys are roughly estimated and show a high security level.