Synaptomys is a genus of North American lemmings. These animals live in wet forested and open areas. They are small, cylindrical rodents with large heads and short ears, legs, and tails. They eat green vegetation such as grasses and sedges. They are often found in colonies.

The genus Synaptomys comprises two extant species and 10 extinct species. S. borealis is sometimes placed in the genus Mictomys.

A number of other fossil species have been included here but have since been transferred to other genera, such as Mictomys and Praesynaptomys.