Master–slave morality (German: Herren- und Sklavenmoral) is a central theme of Friedrich Nietzsche's works, particularly in the first essay of his book On the Genealogy of Morality.

Nietzsche argues that there are two fundamental types of morality: "master morality" and "slave morality", which correspond, respectively, to the dichotomies of "good/bad" and "good/evil". In master morality, "good" is a self-designation of the aristocratic classes; it is synonymous with nobility and everything powerful and life-affirming. "Bad" has no condemnatory implication, merely referring to the "common" or the "low" and the qualities and values associated with them, in contradistinction to the warrior ethos of the ruling nobility. In slave morality, the meaning of "good" is made the antithesis of the original aristocratic "good", which itself is relabeled "evil". This inversion of values develops out of the ressentiment the weak feel toward the powerful.

For Nietzsche, a morality is inseparable from the culture that values it, meaning that each culture's language, codes, practices, narratives, and institutions are informed by the struggle between these two moral structures.

Nietzsche defined master morality as the morality of the dominant, aristocratic warrior-rulers, especially in ancient societies. He criticizes the view (which he identifies with contemporary British ideology) that good is everything that is helpful, and bad is everything that is harmful. He writes that in the prehistoric state "the value or non-value of an action was derived from its consequences" but that ultimately "[t]here are no moral phenomena at all, only moral interpretations of phenomena." According to Nietzsche, in its original meaning, good meant noble, strong, and powerful, while the "bad" is the weak, cowardly, timid, and petty.

For Nietzsche, the essence of master morality is nobility, that is, the qualities typical of and suitable to the dominant warrior-aristocrats. Other qualities that are often valued in master morality are open-mindedness, courage, truthfulness, trustworthiness, and an accurate sense of one's self-worth. Master morality begins in the "noble man", with a spontaneous idea of the good; then the idea of bad develops as what is not good. "The noble type of man experiences itself as determining values; it does not need approval; it judges, 'what is harmful to me is harmful in itself', it knows itself to be that which first accords honour to things; it is value-creating." In other words, the masters value as inherently good all those qualities which, when possessed by them, increase their power or feeling of power, and as bad all those qualities which, if possessed by them, would reduce their power. The nobles define the good based on whether the existence of that quality within them helps the further development of his personal excellence.

In short, master morality identifies "good" with "powerful", to the point that even enemies were considered "good" if they were powerful (and a noble man knows he needs enemies and rivals in order to become stronger). For instance, Nietzsche argues that in the Iliad, both the Trojan heroes and the Greek heroes considered each other as "good", even though they were fighting on opposite sides.

While the nobles did feel there existed mutual obligations among peers (fellow aristocrats), they did not have a concept of "evil" per se: if one of their own acted outside that boundary, they were perceived to be mad (possibly a madness caused by the gods, as is often the case in the Green mythos), rather than worthy of moral blame: the other members could imprison or kill him as a matter of self-preservation, but was not held responsible for his action.

According to Nietzsche, masters create morality; slaves respond to master morality with their slave morality. Unlike master morality, which is sentiment, slave morality is based on ressentiment—devaluing what the master values and what the slave does not have. As master morality originates in the strong, slave morality originates in the weak. Because slave morality is a reaction to oppression, it vilifies its oppressors. Slave morality is the inverse of master morality. As such, it is characterized by pessimism and cynicism. Slave morality is created in opposition to what master morality values as good; it starts by defining everything the master values as "evil" and defines "good" as the lack of "evil".