Casterton is a small village and civil parish close to Kirkby Lonsdale on the River Lune in the south east corner of Cumbria, England. In the 2001 census the parish had a population of 500, decreasing at the 2011 census to 425. The parish is bounded by Kirkby Lonsdale, Barbon, Dent, Leck and Burrow-with-Burrow, and lies just inside the western edge of the Yorkshire Dales National Park: much of the Three Counties System, the longest explored natural cave system in the country, lies beneath it.