The Protestant work ethic, also known as the Calvinist work ethic or the Puritan work ethic, is a work ethic concept in sociology, economics, and history. It emphasizes that a person's subscription to the values espoused by the Protestant faith, particularly Calvinism, results in diligence, discipline, and frugality.

In 1905, sociologist Max Weber initially coined the phrase in his book The Protestant Ethic and the Spirit of Capitalism. Weber asserted that Protestant ethics and values, along with the Calvinist doctrines of asceticism and predestination, enabled the rise and spread of capitalism. Just as priests and caring professionals are deemed to have a vocation (or "calling" from God) for their work, according to the Protestant work ethic the "lowly" workman also has a noble vocation which he can fulfill through dedication to his work.

However, despite Weber's book being one of the most influential and cited in sociology, the thesis presented has been controversial since its release. In opposition to Weber, historians such as Fernand Braudel and Hugh Trevor-Roper assert that the Protestant work ethic did not create capitalism, and that capitalism developed in pre-Reformation Catholic communities. Historian Laurence R. Iannaccone has written that "the most noteworthy feature of the Protestant Ethic thesis is its absence of empirical support."

The concept often helps define the self-view of societies of Northern, Central and Northwestern Europe and the United States.

Following a study trip to America, Weber developed his theory of the Protestant Ethic, which included more considerations than attitude to work. Those with the ethic believed that good fortune (e.g. from hard work) is a vindication of God in this life: a theodicy of fortune; this supported religious and social actions that proved their right to possess even greater wealth. In contrast, those without the ethic instead emphasized that God will be vindicated by granting wealth and happiness in the next life: a theodicy of misfortune. These beliefs guided expectations, behaviour and culture, etc.

According to the theory, Protestants, beginning with Martin Luther, conceptualized worldly work as a duty which benefits both the individual and society. Thus, the Catholic idea of good works was transformed for Protestants into an obligation to consistently work diligently as a sign of grace. Whereas Catholicism teaches that good works are required of Catholics as a necessary manifestation of the faith they received, and that faith apart from works is dead and barren, the Calvinist theologians taught that only those who were predestined to be saved would be saved.

For Protestants, salvation is a gift from God; this is the Protestant distinction of sola gratia. In light of salvation being a gift of grace, Protestants viewed work as stewardship given to them. Thus Protestants were not working in order to achieve salvation but viewed work as the means by which they could be a blessing to others. Hard work and frugality were thought to be two important applications of being a steward of what God had given them. Protestants were thus attracted to these qualities and strove to reach them.

There are many specific theological examples in the Bible used to support a work ethic. Old Testament examples abound, such as God's command in Exodus 20:8–10 to "Remember the Sabbath day, to keep it holy. Six days you shall labor, and do all your work, but the seventh day is a Sabbath to the Lord your God." Another passage from the Book of Proverbs in the Old Testament provides an example: "A little sleep, a little slumber, a little folding of the hands to rest, and poverty will come upon you like a robber, and want like an armed man."