The Heart River flows eastward through the upper part of Grant County, and Cedar Creek flows east-northeastward along the county's southern boundary line.
The county terrain consists of isolated hills among rolling hills, carved by drainages.
The semi-arid ground is partially devoted to agriculture.
The terrain slopes to the east and south; its highest point is a rise near its SW corner, at 2,680' (817m) ASL.
The county has a total area of 1,666 square miles (4,310 km²), of which 1,659 square miles (4,300 km²) is land and 6.8 square miles (18 km²) (0.4%) is water.
Lake Tschida, a Bureau of Reclamation reservoir and recreation area on the Heart River, is the county's largest body of water.

The SW corner of North Dakota observes Mountain Time (Adams, Billings, Bowman, Golden Valley, Grant, Hettinger, Slope, and Stark counties).
The counties of McKenzie, Dunn, and Sioux counties are split, with the western portions observing Mountain Time.
The Heart River flows eastward through the upper part of Grant County, and Cedar Creek flows east-northeastward along the county's southern boundary line. The county terrain consists of isolated hills among rolling hills, carved by drainages. The semi-arid ground is partially devoted to agriculture. The terrain slopes to the east and south; its highest point is a rise near its SW corner, at 2,680' (817m) ASL. The county has a total area of 1,666 square miles (4,310 km²), of which 1,659 square miles (4,300 km²) is land and 6.8 square miles (18 km²) (0.4%) is water. Lake Tschida, a Bureau of Reclamation reservoir and recreation area on the Heart River, is the county's largest body of water.
The SW corner of North Dakota observes Mountain Time (Adams, Billings, Bowman, Golden Valley, Grant, Hettinger, Slope, and Stark counties). The counties of McKenzie, Dunn, and Sioux counties are split, with the western portions observing Mountain Time.
Grant County, North Dakota