In a large, heavy saucepan over moderate heat, cook the sugar, butter, and water, stirring occasionally, until the mixture becomes a golden-brown syrup, about 25 minutes. Remove to a cool surface.
Stir in the peanuts and pour mixture into the pan, spreading the nuts evenly.
Let peanut brittle set and cool before breaking into bite-size pieces. It can be stored in an airtight container in a cool, dry place for up to 1 month.