Diadiaphorus is an extinct genus of litoptern mammal from the Miocene of Argentina (Ituzaingó, Pinturas, Chiquimil and Santa Cruz Formations) and Bolivia (Nazareno Formation), South America.

Diadiaphorus closely resembled a horse, but was only around 1.2 metres (3.9 ft) in body length with a weight 70 kilograms (150 lb), similar to a modern sheep.

Unlike horses, however, Diadiaphorus lacked fused limb bones.

Its skull was short and had a relatively large brain cavity.

Judging from its low molars, Diadiaphorus ate soft vegetation, such as leaves.