社会达尔文主义是将达尔文进化论中自然选择的思想应用于人类社会的一种社会理论。最早提出这一思想的是英国哲学家、作家赫伯特·斯宾塞。社会达尔文主义的风行从19世纪持续到第二次世界大战结束，有人认为现代的社会生物学也可归类到社会达尔文主义学派中。“社会达尔文主义”一词最早出现在美国历史学家理乍得·霍夫施塔特于1944年初版的著作《美国思想中的社会达尔文主义》，所以用“社会达尔文主义”一词来指称1944年前的相关思潮是不确切的，但这种用法已被历史学界广泛采用。

社会达尔文主义曾被其拥护者用来为社会不平等、种族主义和帝国主义正名，理由是赫伯特·斯宾塞所说的“适者生存”。至此，赫伯特·斯宾塞对社会和道德机制进化的理解被异化为与其哲学思想相对立的东西。社会达尔文主义本身并不是一种政治倾向。有的社会达尔文主义者用这一思想说明社会进步和变革的不可避免，也有的社会达尔文主义者认为人类的退化不可避免。社会达尔文主义和进化论一样，经常被卷入关于优生学的争论。

Social Darwinism is a name given to various theories of society which emerged in the United Kingdom, North America, and Western Europe in the 1870s, and which claim to apply biological concepts of natural selection and survival of the fittest to sociology and politics. According to their critics, at least, social Darwinists argue that the strong should see their wealth and power increase while the weak should see their wealth and power decrease. Different social-Darwinist groups have differing views about which groups of people are considered to be the strong and which groups of people are considered to be the weak, and they also hold different opinions about the precise mechanisms that should be used to reward strength and punish weakness. Many such views stress competition between individuals in laissez-faire capitalism, while others are claimed to have motivated ideas of authoritarianism, eugenics, racism, imperialism, fascism, Nazism, and struggle between national or racial groups.