A farmer is a person engaged in agriculture, raising living organisms for food or raw materials. The term usually applies to people who do some combination of raising field crops, orchards, vineyards, poultry, or other livestock. A farmer might own the farmland or might work as a laborer on land owned by others. In most developed economies, a "farmer" is usually a farm owner (landowner), while employees of the farm are known as farm workers (or farmhands). However, in other older definitions a farmer was a person who promotes or improves the growth of plants, land, or crops or raises animals (as livestock or fish) by labor and attention.

Over half a billion farmers are smallholders, most of whom are in developing countries and who economically support almost two billion people. Globally, women constitute more than 40% of agricultural employees.

Farming dates back as far as the Neolithic, being one of the defining characteristics of that era. By the Bronze Age, the Sumerians had an agriculture specialized labor force by 5000–4000 BCE, and heavily depended on irrigation to grow crops. They relied on three-person teams when harvesting in the spring. The Ancient Egypt farmers farmed and relied and irrigated their water from the Nile.

Animal husbandry, the practice of rearing animals specifically for farming purposes, has existed for thousands of years. Dogs were domesticated in East Asia about 15,000 years ago. Goats and sheep were domesticated around 8000 BCE in Asia. Swine or pigs were domesticated by 7000 BCE in the Middle East and China. The earliest evidence of horse domestication dates to around 4000 BCE.

In the US of the 1930s, one farmer could produce only enough food to feed three other consumers. A modern farmer produces enough food to feed well over a hundred people. However, some authors consider this estimate to be flawed, as it does not take into account that farming requires energy and many other resources which have to be provided by additional workers, so that the ratio of people fed to farmers is actually smaller than 100 to 1.

More distinct terms are commonly used to denote farmers who raise specific domesticated animals. For example, those who raise grazing livestock, such as cattle, sheep, goats and horses, are known as ranchers (U.S.), graziers (Australia & UK) or simply stockmen. Sheep, goat and cattle farmers might also be referred to, respectively, as shepherds, goatherds and cowherds. The term dairy farmer is applied to those engaged primarily in milk production, whether from cattle, goats, sheep, or other milk producing animals. A poultry farmer is one who concentrates on raising chickens, turkeys, ducks or geese, for either meat, egg or feather production, or commonly, all three. A person who raises a variety of vegetables for market may be called a truck farmer or market gardener. Dirt farmer is an American colloquial term for a practical farmer, or one who farms his own land.

In developed nations, a farmer (as a profession) is usually defined as someone with an ownership interest in crops or livestock, and who provides land or management in their production. Those who provide only labor are most often called farmhands. Alternatively, growers who manage farmland for an absentee landowner, sharing the harvest (or its profits) are known as sharecroppers or sharefarmers. In the context of agribusiness, a farmer is defined broadly, and thus many individuals not necessarily engaged in full-time farming can nonetheless legally qualify under agricultural policy for various subsidies, incentives, and tax deductions.

In the context of developing nations or other pre-industrial cultures, most farmers practice a meager subsistence agriculture—a simple organic-farming system employing crop rotation, seed saving, slash and burn, or other techniques to maximize efficiency while meeting the needs of the household or community. One subsisting in this way may become labelled as a peasant, often associated disparagingly with a "peasant mentality".