There is disclosed a biomechanically driven prosthetic thumb assembly. Embodiments include an H-shaped rocker and a distal ring configured to receive a user's residual thumb, the distal ring and the rocker each independently and rotatively coupled between a coupling tip and a proximal anchor plate configured for affixing to a hand strap secured to a user. The coupling tip is articulated in response to a pulling force of the H-shaped rocker. Additional embodiments include a bidirectional thumb assembly including a ring mounted upon an adjustable ring tendon that is rotatively coupled between a coupling tip and a proximal anchor plate, which is rotatively coupled with a hand strap attached to the user. Vertical movement of the residual thumb within the ring actuates the coupling tip within a vertical plane. Lateral movement of the residual thumb within the ring actuates the coupling tip within a lateral plane. Other embodiments are disclosed.