Public interest law refers to legal practices undertaken to help poor or marginalized people, or to effect change in social policies in the public interest, on 'not for profit' terms (pro bono publico).

It is not a body of law or a legal field. Rather, it denotes the clientele they represent. Instead of serving powerful economic interests, it stands for the advocacy of otherwise under-represented or vulnerable individuals, especially those living in poverty. It has grown to encompass a broader range of activities, typically the field of non-lawyers like civil rights, civil liberties, women's rights, consumer rights, environmental protection, and so on. Nevertheless, a common ethic for public-interest lawyers in a growing number of countries remains "fighting for the little guy".

By jurisdiction.

"Public interest law" is a term that became widely adopted in the United States during and after the social turmoil of the 1960s. It built on a tradition exemplified by Louis Brandeis, who before becoming a U.S. Supreme Court justice incorporated advocacy for the interests of the general public into his legal practice. In a celebrated 1905 speech, Brandeis decried the legal profession, complaining that "able lawyers have to a large extent allowed themselves to become adjuncts of great corporations and have neglected their obligation to use their powers for the protection of the people." In the late 1960s and 1970s, large numbers of American law school graduates began to seek "relevance" in their work—wishing to affect the social issues that were so visibly and hotly debated within American society at that time. They defined themselves as public interest lawyers in order to distinguish themselves from the "corporate adjuncts" referred to by Brandeis.

Summing up the movement's history in the United States, Stanford University Law Professor Deborah Rhode writes: "Public interest lawyers have saved lives, protected fundamental rights, established crucial principles, transformed institutions, and ensured essential benefits for those who need them most....In virtually every major American social reform movement of the last half century, public interest lawyers have played an important role."

Public interest law is institutionalized in the United States. Nongovernmental organizations that work to promote and protect human rights using the U.S. legal system, fight to protect the environment, or advocate on behalf of consumers, call themselves public interest law organizations. A large community of lawyers’ practices public interest law in the form of providing legal aid free of charge to those who cannot afford to pay for it. However, the grim reality remains that lawyers are underpaid and grossly overworked, offering perfunctory representation. Clinical legal education, which is well established in the United States, provides opportunities for law students to do practical legal work on basic legal matters as well as more complex public interest issues, such as women's rights, anti-discrimination law, constitutional rights, and environmental protection, among others. Some law schools have public interest law centers, which advise law students interested in pursuing public interest law careers. Pro bono programs at bar associations and law firms provide opportunities for commercial lawyers to donate time to public interest law activities.