Extraterritorial jurisdiction (ETJ) is the legal ability of a government to exercise authority beyond its normal boundaries.

Any authority can claim ETJ over any external territory they wish. However, for the claim to be effective in the external territory (except by the exercise of force), it must be agreed either with the legal authority in the external territory, or with a legal authority that covers both territories. When unqualified, ETJ usually refers to such an agreed jurisdiction, or it will be called something like "claimed ETJ".

The phrase may also refer to a country's laws extending beyond its boundaries in the sense that they may authorise the courts of that country to enforce their jurisdiction against parties appearing before them in with respect to acts they allegedly engaged in outside that country. This does not depend on the co-operation of other countries, since the affected people are within the relevant country (or at least, in a case involving a person being tried in absentia, the case is being heard by a court of that country). For example, many countries have laws which give their criminal courts jurisdiction to try prosecutions for piracy, sexual offences against children, computer crimes and/or terrorism committed outside their national boundaries. Sometimes such laws only apply to nationals of that country, and sometimes they may apply to anyone.