On Liberty is a philosophical work by 19th century English philosopher John Stuart Mill, first published in 1859. To the Victorian readers of the time it was a radical work, advocating moral and economic freedom of individuals from the state.

Perhaps the most memorable point made by Mill in this work, and his basis for liberty, is that "Over himself, over his own body and mind, the individual is sovereign". Mill is compelled to say this in opposition to what he calls the "tyranny of the majority", wherein through control of etiquette and morality, society is an unelected power that can do horrific things. Mill's work could be considered a reaction to this social control by the majority and his advocacy of individual decision-making over the self. The famous 'Harm Principle' is also articulated in this work: people can do anything they like as long as it does not harm others. All branches of liberalism—as well as other political ideologies—consider this to be one of their core principles. However, they often disagree on what exactly constitutes harm.

On Liberty was an enormously influential work; the ideas presented within it remain the basis of much political thought since. Aside from the popularity of the ideas themselves, it is quite short and its themes easily accessible to a non-expert. It has remained in print continuously since its initial publication. To this day, a copy of On Liberty has been passed to the president of the British Liberals and then Liberal Democrats as a symbol of office and succession from the party that Mill helped found.