Longtermism is the ethical view that positively influencing the long-term future is a key moral priority. It is an important concept in effective altruism and a primary motivation for efforts that aim to reduce existential risks to humanity.

The key argument for longtermism has been summarized as follows: "future people matter morally just as much as people alive today; ... there may well be more people alive in the future than there are in the present or have been in the past; and ... we can positively affect future peoples' lives." These three ideas taken together suggest, to those advocating longtermism, that it is the responsibility of those living now to ensure that future generations get to survive and flourish.

Philosopher William MacAskill defines longtermism as "the view that positively influencing the longterm future is a key moral priority of our time". He distinguishes it from strong longtermism, "the view that positively influencing the longterm future is the key moral priority of our time".

In his book The Precipice: Existential Risk and the Future of Humanity, philosopher Toby Ord describes longtermism as follows: "longtermism ... is especially concerned with the impacts of our actions upon the longterm future. It takes seriously the fact that our own generation is but one page in a much longer story, and that our most important role may be how we shape—or fail to shape—that story. Working to safeguard humanity's potential is one avenue for such a lasting impact and there may be others too." In addition, Ord notes that "longtermism is animated by a moral re-orientation toward the vast future that existential risks threaten to foreclose."

Because it is generally infeasible to use traditional research techniques such as randomized controlled trials to analyze existential risks, researchers such as Nick Bostrom have used methods such as expert opinion elicitation to estimate their importance. Ord offered probability estimates for a number of existential risks in The Precipice.

The term "longtermism" was coined around 2017 by Oxford philosophers William MacAskill and Toby Ord. The view draws inspiration from the work of Nick Bostrom, Nick Beckstead, and others. While its coinage is relatively new, some aspects of longtermism have been thought about for centuries. The oral constitution of the Iroquois Confederacy, the Gayanashagowa, encourages all decision-making to “have always in view not only the present but also the coming generations”. This has been interpreted to mean that decisions should be made so as to be of benefit to the seventh generation in the future. These ideas have re-emerged in contemporary thought with thinkers such as Derek Parfit in his 1984 book Reasons and Persons, and Jonathan Schell in his 1982 book The Fate of the Earth.

Longtermist ideas have given rise to a community of individuals and organizations working to protect the interests of future generations. Organizations working on longtermist topics include Cambridge University's Centre for the Study of Existential Risk, the Future of Life Institute, the Global Priorities Institute, the Stanford Existential Risks Initiative, 80,000 Hours, Open Philanthropy, The Forethought Foundation, and Longview Philanthropy.

Researchers studying longtermism believe that we can improve the long-term future in two ways: "by averting permanent catastrophes, thereby ensuring civilisation’s survival; or by changing civilisation’s trajectory to make it better while it lasts. Broadly, ensuring survival increases the quantity of future life; trajectory changes increase its quality".