A migrant worker is a person who migrates within a home country or outside it to pursue work. Migrant workers usually do not have an intention to stay permanently in the country or region in which they work.

Migrant workers who work outside their home country are also called foreign workers. They may also be called expatriates or guest workers, especially when they have been sent for or invited to work in the host country before leaving the home country.

The International Labour Organization estimated in 2019 that there were 169 million international migrants worldwide. Some countries have millions of migrant workers. Some migrant workers are illegal immigrants or slaves.

The International Organization for Migration's Global Migration Data Analysis Centre states that "there is no internationally accepted statistical definition of labour migration", but refers to the International Labour Organization (ILO)'s definition: "international migrants who are currently employed or unemployed and seeking employment in their present country of residence".

An estimated 14 million foreign workers live in the United States, which draws most of its immigrants from Mexico, including 4 or 5 million illegal workers. It is estimated that around 5 million foreign workers live in northwestern Europe, half-a-million in Japan, and 5 million in Saudi Arabia. A comparable number of dependents are accompanying international workers.

There are approximately 14 million non-permanent workers in the United States. Today it is estimated that there are about 10.7 million undocumented migrant workers in the United States, many of whom come from Mexico and other countries in Central America. These workers often travel to the United States to look for occupational opportunities and to provide economic stability for their families. These workers risk their lives to cross the border - facing extreme heat and the risk of being caught by border control to find jobs in the United States, often facing language barriers, cultural shock, stigma, and discrimination upon reaching the United States. In the United States 65% of farm workers are Latino, with about half of the workers being undocumented.

The jobs available to undocumented migrant workers in America are often in the domestic, industrial and agricultural field. These jobs are often physically demanding and are often dangerous. All of these workers are exposed to fast paces, and repetitive motions at work which can easily lead to injury. In the domestic field many, often female, migrant workers are subject to physically demeaning cleaning jobs that involve harsh chemical cleaners that have adverse health effects. Industrial workers often have to use heavy machinery without proper training or safety protocols. In the agricultural field, the workers spend long days doing intense physical labor often in extreme weather conditions such as storms and heat. The workers are also exposed to unsafe levels of pesticides, herbicides, and fertilizers, which can lead to respiratory illness, dermatitis, cancer, and reproductive problems. If these workers do get injured or sick from work, they are left to navigate the American healthcare system without insurance and with a language barrier. The undocumented nature of many migrant workers in the United States leads to the exploitation of the population as a whole. The lack of government regulation of employers that hire migrant workers means that the workers work long hours in adverse conditions for little pay. These workers often live in unsafe living conditions that are overcrowded and often unsanitary. Additionally, they are at constant risk of sudden deportation, which leads to migrant workers accepting poor working conditions and unlivable wages from their employers. This leads to employers exploiting migrant workers by subjecting them to unhealthy working conditions for poverty wages. Socially, migrant workers face social stigma and discrimination as they are referred to as "illegal migrants".

Additionally migrant workers are one of the most vulnerable groups in regards to the impending climate crisis. These workers are already exposed to long work days in extreme conditions. The nature of agricultural work, industrial, and construction work puts these migrant workers at higher rates of heat stroke, sun exposure, and injuries caused by storm conditions. The climate crisis will disproportionately affect migrant workers because of the intensive outdoor nature of their work and the lack of access they have to social resources.