Place sugar in medium saucepan set over medium heat. Without stirring, let sugar melt until it's completely liquid and a light amber color.
Remove pan from heat and pour in the warm water. The mixture will sizzle and bubble and some of the sugar may harden. if this happens, return saucepan to heat and simmer until mixture is liquid again.
Pour syrup in jar or heat-proof bowl, mix in pumpkin pie spice, and let cool completely before using in cocktail.