In short, the term Leptocleidus means "slender clavicle". It comes from a merge of the Greek words λεπτοσ, meaning "slender" and κλειδ (also spelled κλεισ) meaning clavicle.
With large clavicles and interclavicle and small scapulae, Leptocleidus resembled the Early Jurassic Rhomaleosaurus and members of the Cretaceous family, Polycotylidae. The animal had 21 teeth on either side of its maxilla and approximately 35 teeth on each side of the mandible. The Leptocleidus's triangle-shaped skull had a crest running from a ridge on the end of the nose to the nasal region. Differing from other pliosaurids, Leptocleidus had single-headed cervical ribs and a deep depression in the centra of the neck vertebrae. Leptocleidus was on an average of 3 meters (10 feet) long. Leptocleidus superstes however, was found to be almost 50% smaller (1.5m, 5 ft) making it the smallest known species.