Hemp (Cannabis sp.) has been a fundamental plant for the development of human societies. Its fibers have long been used for textiles and rope making, which requires prior stem retting. This process is essential for extracting fibers from the stem of the plant, but can adversely affect the quality of surface waters. The history of human activities related to hemp (its domestication, spread, and processing) is frequently reconstructed from seeds and pollen detected in archaeological sites or in sedimentary archives, but this method does not always make it possible to ascertain whether retting took place. Hemp is also known to contain phytocannabinoids, a type of chemicals that is specific to the plant. Here we report on the detection of one of these chemicals, cannabinol (CBN), preserved in a sediment record from a lake in the French Massif Central covering the past 1800 yr. The presence of this molecule in the sedimentary record is related to retting. Analysis of the evolution of CBN concentrations shows that hemp retting was a significant activity in the area until ca. A.D. 1850. These findings, supported by pollen analyses and historical data, show that this novel sedimentary tracer can help to better constrain past impacts of human activities on the environment.