Judicial activism is a judicial philosophy holding that courts can and should go beyond the applicable law to consider broader societal implications of their decisions. It is sometimes used as an antonym of judicial restraint. The term usually implies that judges make rulings based on their own views rather than on precedent. The definition of judicial activism and the specific decisions that are activist are controversial political issues. The question of judicial activism is closely related to judicial interpretation, statutory interpretation, and separation of powers.

Arthur Schlesinger Jr. introduced the term "judicial activism" in a January 1947 Fortune magazine article titled "The Supreme Court: 1947".

The phrase has been controversial since its beginning. An article by Craig Green, "An Intellectual History of Judicial Activism," is critical of Schlesinger's use of the term; "Schlesinger's original introduction of judicial activism was doubly blurred: not only did he fail to explain what counts as activism, he also declined to say whether activism is good or bad."

Even before this phrase was first used, the general concept already existed. For example, Thomas Jefferson referred to the "despotic behaviour" of Federalist federal judges, in particular Chief Justice John Marshall.

Black's Law Dictionary defines judicial activism as a "philosophy of judicial decision-making whereby judges allow their personal views about public policy, among other factors, to guide their decisions."

Political science professor Bradley Canon has posited six dimensions along which judge courts may be perceived as activist: majoritarianism, interpretive stability, interpretive fidelity, substance/democratic process, specificity of policy, and availability of an alternate policymaker.

David A. Strauss has argued that judicial activism can be narrowly defined as one or more of three possible actions: overturning laws as unconstitutional, overturning judicial precedent, and ruling against a preferred interpretation of the constitution.

Others have been less confident of the term's meaning, finding it instead to be little more than a rhetorical shorthand. Kermit Roosevelt III has argued that "in practice 'activist' turns out to be little more than a rhetorically charged shorthand for decisions the speaker disagrees with". Roosevelt defines judicial activism as "an approach to the exercise of judicial review, or a description of a particular judicial decision, in which a judge is generally considered more willing to decide constitutional issues and to invalidate legislative or executive actions."; likewise, the solicitor general under George W. Bush, Theodore Olson, said in an interview on Fox News Sunday, with regard to a case for same-sex marriage he had successfully litigated, that "most people use the term 'judicial activism' to explain decisions that they don't like." Supreme Court Justice Anthony Kennedy said that, "An activist court is a court that makes a decision you don't like."