On Contradiction (simplified Chinese: 矛盾论; traditional Chinese: 矛盾論; pinyin: Máodùn Lùn; lit. 'To discuss contradiction') is a 1937 essay by the Chinese Communist revolutionary Mao Zedong. Along with On Practice it forms the philosophical underpinnings of the political ideology that would later become Maoism. It was written in August 1937, as an interpretation of the philosophy of dialectical materialism, while Mao was at his guerrilla base in Yan'an. Mao suggests that all movement and life is a result of contradiction. Mao separates his paper into different sections: the two world outlooks, the universality of contradiction, the particularity of contradiction, the principal contradiction and principal aspect of contradiction, the identity and struggle of aspects of contradiction, the place of antagonism in contradiction, and finally the conclusion. Mao further develops the theme laid out in On Contradiction in his 1957 speech On the Correct Handling of Contradictions Among the People.

Mao describes existence as being made up of constant transformation and contradiction. Nothing is constant as in metaphysics and can only exist based on opposing contradictions. He uses the concept of contradiction to explain different Chinese historical time periods and social events. Mao's form of talking about contradiction creates a modified concept that brought forth the ideal of Chinese Marxism. This text continues to influence and educate Chinese Marxists.