Saltpeter is an Alchemy ingredient that is primarily used in the crafting of Bombs.

Saltpeter can be tough to find early in the game, as it may not always be for sale from merchants. It can sometimes be found in random containers, but for a surefire way to obtain some, complete the Side Quest: Contract: Devil by the Well. When you turn in the quest, ask the man if he knows about the name from the journal, and he will direct you to the herbalist. If you tell the herbalist, Tomira, what you know, she will reward you with Saltpeter and other ingredients. She also have Saltpeter available for sale.