Castoreum is a yellowish exudate from the castor sacs of mature beavers. Beavers use castoreum in combination with urine to scent mark their territory. Both beaver sexes have a pair of castor sacs and a pair of anal glands, located in two cavities under the skin between the pelvis and the base of the tail. The castor sacs are not true glands (endocrine or exocrine) on a cellular level, hence references to these structures as preputial glands, castor glands, or scent glands are misnomers.

It is used as a tincture in some perfumes and was sometimes used as a food additive in the early 1900s.

The sacs brought C$92–$180/kg ($2.62–$5.10/oz) when auctioned at the May–June 2016 North American Fur Auction.