Cyamodus (pron.: SIE-ah-MO-dus) is a genus of placodonts known from several species described from Middle-Late Triassic of Europe and China. The genus was described by Christian Erich Hermann von Meyer in 1863, based on specimens found in Germany. Like some other placodonts, Cyamodus has an armoured carapace composed of irregular hexagonal plates, with the mouth containing a small number of large, rounded teeth that were likely involved in crushing hard shelled organisms (durophagy).

Thus far, six species of Cyamodus have been identified - C. rostratus, C. munsteri, C. tarnowitzensis, C. hildegardis, C. kuhnschneyderi, and C. orientalis.

Initially considered to be an ancestral turtle due to its testudine-like head and large, bifurcated carapace. However further investigation resulted in its reclassification as a placodont, and it is closely related to other turtle-like reptiles of the Triassic period such as Henodus and Psephoderma. Similar to these other placodonts, Cyamodus lived hovering close to the sea floor, vacuuming up various shellfish, and crushing them between its blunt teeth.

Historically, the first Cyamodus remains were found in Upper Muschelkalk shallow marine limestones at near Bayreuth in Bavaria (Germany). They included the incomplete holotype skulls of Cyamodus muensteri and Cyamodus rostatus, which along with all other placodont remains recovered from the six quarries on the Lainecker Range in northern Bavaria were originally considered to have been derived from fish. The earliest Cyamodus skull was later restored by Muenster, with the addition of four teeth that had not been present in the original skull, and was named Placodus muensteri.

Further placodont remains were found by Muenster, who collected many placodont cranial remains in the Bindlach and Lainecker Range quarries. All placodont remains from those sites were then revised as being of reptilian origin by Owen (1858). A complete Cyamodus skeleton, including its skull, is known for C. hildegardis, which was found outside the Germanic Basin in the northern Tethys in Switzerland. Middle Triassic sauropterygian placodonts have become increasingly important for developing new ideas to the evolutionary history of their relatives, the turtles, whereas modern analyses place placodonts not as their ancestors using morphological cladistic analyses based on the bone osteology. The study of these placodonts contributes to our understanding of the Germanic Basin and the reptile distributions.

An intriguing placodont that appears to be intermediate between Cyamodus and the placochelyids, Protenodontosaurus italicus, was described by Giovanni Pinna in 1990.

Cyamodus was a relatively small reptile, with most species measuring 1.3–1.5 m (4.3–4.9 ft) long and weighing 20–25 kg (44–55 lb); the smallest species, C. rostratus, was about 0.9 m (3.0 ft) long and weighed 5 kg (11 lb). It was a heavily armored swimmer that fed mainly on shellfish that it was specialized to uproot and crush with its powerful jaws. The body of Cyamodus, specifically the armor, has been described as possessing a turtle-like flatness. The shell was a two-part carapace on the upper surface of the body. The larger half covered Cyamodus from the neck to the hips and spread out flat, almost encompassing the limbs. The second, smaller plate covered the hips and the base of the tail. The shells themselves are covered in hexagonal or circular plates of armor. The skull is heart-shaped and broad.

Distinct from Paraplacodus, the skull of Cyamodus had a shorter rostrum, a smaller orbit and a larger upper temporal fenestra that was rimmed by ossifications. The teeth were flat discs, only one tooth appeared on each premaxilla and only two teeth appeared on each maxilla, with the largest teeth on the pterygoid. The quadratojugal joined the squamosal and sealed up the lateral temporal fenestra from the ancestral species (Paraplacodus).