Rümmler's coccymys is a small mouse recorded in the mountains of western New Guinea. It has soft, dark brown fur and a long tail.
The back is dark brown, while the underside is grayish white to gray. The tail is long and brown.
The paper describes Coccymys as nocturnal and able to climb. For this species, the authors infer that it may also use the ground and low vegetation from its morphology.
An extinction-risk assessment has not been added yet.