Baryonyx, whose name means "heavy claw," is a genus represented by a single species of spinosaurid theropod dinosaur that lived in the mid-Cretaceous period, approximately 125 million years ago during the Aptian, in what is now Europe and possibly Africa.

Baryonyx was a large predator that measured about 9.75 meters in length, 2.55 meters in height at the hip, and weighed between 1.7 and 2.7 tons. Approximate dimensions have long varied, but with the latest technology, the previous dimensions were estimated.

Baryonyx is a very unusual theropod. The structure of its pelvis suggests that it was bipedal when moving from one place to another. Like dromaeosaurids, Baryonyx had a pair of large, curved claws, about 35 centimeters long, but unlike dromaeosaurids, they were not located on the feet, but on the hands.

The skull of Baryonyx is incompletely known, and much of the middle and hind portions are missing. The total skull length has been estimated at 950 millimeters (37.5 in), based on comparison with the related genus Suchomimus, which is 20% larger. It is elongated, and the 170 millimeter (6.7 in) front premaxillae formed a long, low snout with a rounded upper surface.

Baryonyx had a rudimentary secondary palate, similar to crocodiles but unlike most theropod dinosaurs. A rough surface suggests the presence of a horny pad on the roof of the mouth.

The dentary had many foramina, which were passages for nerves and blood vessels. It has been suggested that some of the cranial bones of Baryonyx have been misidentified, resulting in an occipital bone that is too deep in the reconstruction, and that the skull was probably as low, long, and narrow as that of its close relative, Suchomimus. Most of the teeth found with the holotype specimen were not attached to the skull. A few remained in the upper jaw, and only small replacement teeth were in the lower jaw. The teeth were shaped like recurved cones, somewhat flattened laterally.

The scapulae were robust. The forearm bones were short relative to the animal's size, but broad and robust. The humerus was short and robust, with its broad, expanded and flattened ends forming the upper part of the deltopectoral crest, the muscle attachment area, and the lower portion for articulation with the radius and ulna. The radius was short, robust, and straight, and the olecranon of the ulna appeared to be very powerful. The lower part of the ulna was widely expanded. The hands had three fingers, the first finger bearing a large claw formed from the nail bone measuring approximately 31 centimeters along its curve, which would have been even larger thanks to a keratin sheath in life. Aside from its size, the claw's proportions were fairly typical of a theropod. It was bilaterally symmetrical, slightly compressed, smoothly rounded, and pointed. A groove for the sheath ran along the length of the claw. The pubic foot of the pelvis was not expanded.