Blue Earth County is a county located in the U.S. state of Minnesota. As of 2000, the population was 55,941. Its county seat is Mankato. Blue Earth County is part of the Mankato–North Mankato Metropolitan Statistical Area.
According to the U.S. Census Bureau, the county has a total area of 766 square miles (1,984 km˛), of which 752 square miles (1,949 km˛) is land and 14 square miles (35 km˛) (1.77%) is water. The Le Sueur River flows through a part of the county. The land surface is relatively flat with over 30 lakes in the county. In the northeastern part of the county there are many forests that are called big woods. The rivers that flow out of the northeastern part are surrounded by these big woods. Most of the county is grassland prairie but scattered parts of it are wet prairie. Some spots that surround the rivers are oak openings and barren brushland.