Charles Knowlton (May 10, 1800 February 20, 1850) was an American physician and writer. He was an atheist.
In 1832, Knowlton moved his family and medical practice to Ashfield, Massachusetts. A year later, the town's new minister, Mason Grosvenor, began a campaign against "infidelity and licentiousness," targeting Knowlton as its source. Knowlton had written a little book entitled, Fruits of Philosophy, or the Private Companion of Young Married People, and had been showing it to his patients. It contained a summary of what was then known about the physiology of conception, listed a number of methods to treat infertility and impotence, and explained a method of birth control he had developed.