Free immigration or open immigration is the belief that people should be able to migrate to whatever country they chose, free of substantial barriers. Free immigration is similar in spirit to the concept of free trade, and both are advocated by free market economists on the grounds that economics is not a zero-sum game and that free markets are arguably the best way to create a fairer and balanced economic system, thereby increasing the overall economic benefits to all concerned parties. Paradoxically, many libertarians and socialists advocate open immigration, notwithstanding other noteworthy differences between these two political ideologies.

Arguments against free immigration are similar to arguments against free trade, for example, protectionism or xenophobia. Specifically, an influx of cheap labor could easily deflate wages for workers who are already established in a particular labor market, and (at least in the short term) have a negative impact on the standard of living for the more established workers. Other critics of free immigration are concerned that it would be unfair to current homeowners if an influx of new residents greatly brought down the property values and attractiveness of living in that location, or, alternatively, increased the demand to live in the city so much that the home owner would not be able to keep up with increased taxes from higher property values. However, free market economists believe that competition is the essence of a healthy economic system, and that any short-term negative impact on individual economic actors that is caused by free immigration is more than justified by the prospects of long-term growth for the economy as a whole.

War-related chaos can lead to the breakdown of borders and allow for de facto free immigration. The natural attempts to flee strife, or escape a conquering enemy, can quickly lead to millions of refugees. Even where border controls are in place they can be temporarily overwhelmed by the sheer numbers of people. Once settled into refugee camps, these reluctant immigrants may take decades to be either repatriated back or naturalized into their new country. The Cold War saw a migration paradox in which the communist states forbid emmigrants from leaving (on pain of death), while the "Free World" would freely accept the defectors. This policy persists for Cubans and the Hmong, who are both allowed particular forms of free immigration to the United States based on their automatic refugee status.

Until the 20th century, the United States and Canada had policies close to open immigration for Europeans. In the Immigration Act of 1924, the United States Congress adopted strict immigration controls, especially on those coming from outside western Europe. These rules were later relaxed in the 1965 Immigration Reform Act. Currently, to cite two examples, open immigration is allowed within the European Union and between Puerto Rico and the United States. In a limited form of free immigration, the state of Israel extends citizenship to all persons of Jewish heritage throughout the world. Many immigration reductionists and anti-immigrationists in the US are of the opinion that the current mix of liberal legal immigration and massive illegal immigration amounts to open borders.