The county terrain consists of low rolling hills, etched with drainages, completely devoted to agriculture where possible. The terrain slopes to the east and north, with its highest point near the midpoint of its south border, at 1,293 ft ASL. The county has an area of 432 sqmi, of which 430 sqmi are land and 2.7 sqmi (0.6%) are water.

The Straight River flows northward through the county, and the Le Sueur River flows through its southwestern