Mutual Aid: A Factor of Evolution is a book by Peter Kropotkin on the subject of mutual aid, written while he was living in exile in England. It was first published by William Heinemann in London in October 1902. The individual chapters had originally been published in 1890-96 as a series of essays in the British monthly literary magazine, Nineteenth Century.

Written partly in response to Social Darwinism and in particular to Thomas H. Huxley's Nineteenth Century essay, "The Struggle for Existence," Kropotkin's book drew on his experiences in scientific expeditions in Siberia to illustrate the phenomenon of cooperation. After examining the evidence of cooperation in animals, "savages," "barbarians," in medieval cities, and in modern times, he concludes that cooperation and mutual aid are as important in the evolution of the species as competition and mutual strife, if not more so.