The hindflippers of California sea lions (Zalophus californianus) have previously been overlooked as aquatic control surfaces. Although passively trailed and adducted in rectilinear swimming, the hindflippers are abducted during turning maneuvers. As the anatomy of sea lion hindflippers had not previously been described in detail, examination of the anatomy and morphology of the hindflippers were completed via scaled morphological measurements, CT scan, and dissections. Between the adducted and abducted postures of the hindflipper, there is a mean 22.6% increase in planar surface area. Flexible extensions of the digits beyond the distal phalanx gives the trailing edge of the flipper a crenellated appearance. Representing 17.2% of the total length of the hindflipper, the crenellations are composed of a collagenous matrix with no bony elements. The skeletal framework of the hindflipper is similar to that of human feet with modifications that create the characteristic delta shape. While most of the muscle origins and insertions of the hindflipper are similar to those in human feet, major differences exist. Abductor Hallucis and Flexor Digitorum Brevis have extrinsic origins in the hindflipper compared to intrinsic origins in humans. The tendons of Flexor Hallucis Longus and Flexor Digitorum Longus insert into the matrix of the crenellations as opposed to insertions onto the distal phalanges. The tendon morphology of Flexor Digitorum Brevis includes a foramen, through which the tendons of Flexor Digitorum Longus pass. These anatomical differences indicate an increased demand for the ability to abduct the hindflipper and the novel tendon morphologies suggest the possibility of active stiffness modulation of the digits and crenellations, which may assist in aquatic maneuvering performance.