Grundy County is a county located in the U.S. state of Tennessee. It is located in Middle Tennessee, bordering East Tennessee. As of 2021, the population was 13,622. Its county seat is Altamont. The county is named in honor of Felix Grundy.

Middle Tennessee is one of the three Grand Divisions of Tennessee that composes roughly the central portion of the state. It is delineated according to state law as 41 of the state's 95 counties. Middle Tennessee contains the state's capital and largest city, Nashville, as well as Clarksville, the state's fifth largest city, and Murfreesboro, the state's sixth largest city and largest suburb of Nashville. The Nashville metropolitan area, located entirely within the region, is the most populous metropolitan area in the state, and the Clarksville metropolitan area is the state's sixth most populous. Middle Tennessee is both the largest, in terms of land area, and the most populous of the state's three Grand Divisions.