Tremacebus is an extinct genus of New World monkeys from the Early Miocene (Colhuehuapian in the SALMA classification). The type species is T. harringtoni.

Description

Tremacebus was about 1 m in length, and would have resembled a modern night monkey, to which it may have been related, though possibly a stem aotid. However, its eyes appear to have been smaller than the modern species, CT scans of the cranium suggest a relatively small olfactory bulb and poor sense of smell, compared with night monkeys. These features suggest that it may not have been nocturnal. It had an estimated body mass of 1.8 kg. Only a few fossils have been found, including a skull from the Sarmiento Formation, Patagonia.