The Big Snowy Group is a stratigraphical unit of Chesterian age in the Williston Basin.

The Big Snowy Group is composed of three subdivisions, from top to base: [1] The Big Snowy Group reaches a maximum thickness of 135 metres (440 ft) in the Williston Basin.

[1] It is exposed in outcrop in the Big Snowy Mountains, Little Belt Mountains, Castle Mountains and Lombard Hills of central Montana.

It occurs in the sub-surface throughout the central part of the Williston Basin and into a limited area of south-central Saskatchewan.

The Big Snowy Group is unconformably overlain by the Tyler Formation in Montana, and by the Watrous Formation in Saskatchewan; It disconformably overlays the Madison Group.