Thomasomys cinereiventer is a large oldfield mouse recorded in moist montane forests of the western Colombian Andes. Specimens also came from selectively logged cloud forest and nearby fields and pastures, but its ecology remains poorly known.
It is large for its genus, with a very dark brown upper coat, gray underside, small brown ears, and long, soft fur.
Known collection localities are in moist montane forest. Specimens have also been collected in selectively logged cloud forest and adjacent fields and pastures, but these records do not establish habitat preference.
An extinction-risk assessment has not been added yet.