The Demon in the Freezer is a 2002 nonfiction book on the biological weapon agents smallpox and anthrax and how the American government develops defensive measures against them. It was written by journalist Richard Preston, also author of the best-selling book The Hot Zone (1994), about ebolavirus outbreaks in Africa and Reston, Virginia and the U.S. government's response to them.

The book is primarily an account of the Smallpox Eradication Program (1967–1980), the ongoing perception by the U.S. government that smallpox is still a potential bioterrorism agent, and the controversy over whether or not the remaining samples of smallpox virus in Atlanta and Moscow (the "demon" in the freezer) should be finally destroyed. However, the writer was overtaken by events—the 9/11 attacks and the anthrax letter incidents (called "Amerithrax"), both in 2001—and so much of the book interweaves the anthrax investigation with the smallpox material in a manner some critics have said is "awkward" and somewhat "disjointed".