The territorial sea is a belt of coastal waters over which a coastal state exercises sovereignty, subject to important rules of international law. Under UNCLOS, a state may claim a territorial sea extending up to 12 nautical miles from its applicable baselines. The sovereignty extends over the territorial sea itself and also to the airspace above it and the seabed and subsoil below it, subject to the legal regime established by international law.
The territorial sea is therefore much closer to territorial sovereignty than the exclusive economic zone. However, it is not identical to land territory because foreign ships generally possess the right of innocent passage through the territorial sea under the conditions established by international law. The territorial sea is consequently a national maritime zone with significant sovereign authority combined with internationally recognised navigational rights.
The 12 nautical mile limit is measured from the applicable baseline. The normal baseline is generally the low water line along the coast as marked on officially recognised large scale charts, subject to special rules. Straight baselines may be used in certain circumstances, such as deeply indented coasts or coasts fringed with islands, when the requirements of international law are met. Archipelagic states have additional rules concerning archipelagic baselines. Baseline selection is important because all outward maritime zones are measured from it. A coastline that appears simple on a general map can therefore involve detailed legal and technical questions when the precise starting points for maritime measurement are established.
Sovereignty in the territorial sea means that the coastal state can regulate many activities within the zone, including navigation, resource use, environmental protection, customs matters, immigration, pollution control and public safety, subject to international law. The coastal state can enact laws and regulations concerning navigation and other matters within its lawful competence. However, the existence of sovereignty does not mean that foreign vessels can simply be treated as if they were on foreign territory when passing through. International law recognises the right of innocent passage. This creates a balance between coastal state authority and the international interest in maritime navigation. The detailed conditions governing passage, enforcement and suspension are therefore important parts of the territorial sea regime.
Innocent passage is one of the defining features distinguishing the territorial sea from internal waters. A foreign ship may generally pass continuously and expeditiously through the territorial sea as long as its passage is not prejudicial to the peace, good order or security of the coastal state according to the applicable UNCLOS rules. Activities such as certain forms of weapons practice, intelligence gathering, serious pollution, fishing or unauthorised loading and unloading can affect whether passage is considered innocent.
The exact legal assessment depends on the circumstances and applicable law. Coastal states may adopt laws concerning innocent passage in accordance with international requirements. The existence of innocent passage means that sovereignty in the territorial sea is qualified by an internationally recognised navigation regime rather than being identical to unrestricted sovereignty over land.
The territorial sea is distinct from internal waters, the contiguous zone and the EEZ. Internal waters are generally waters on the landward side of the baseline, subject to special rules, and the coastal state normally exercises sovereignty there with a different legal regime for foreign access. The territorial sea extends outward from the baseline to a maximum of 12 nautical miles.
The contiguous zone may extend to 24 nautical miles from the baseline and gives the coastal state specific enforcement powers concerning customs, fiscal, immigration and sanitary laws. The EEZ can extend to 200 nautical miles and primarily concerns resource rights and specified jurisdiction rather than complete sovereignty. These zones form parts of a broader maritime legal system in which the intensity and type of coastal state authority change with distance from the baseline.
Territorial seas can overlap where states are less than 24 nautical miles apart or where their relevant maritime entitlements otherwise intersect. In such cases, the states must determine a maritime boundary through agreement or other applicable legal procedures. The boundary is not necessarily established automatically by simply measuring 12 nautical miles from each coast and selecting the midpoint in every circumstance. Geographic configuration and relevant agreements can affect delimitation. Islands can also influence the maritime boundary according to their legal status and geographical circumstances. A maritime boundary dispute can therefore involve detailed questions about baselines, islands, coastal configuration and the applicable principles of international law.
The territorial sea has major practical significance for navigation, security, environmental protection, fisheries, law enforcement and offshore activity. Coastal states regulate shipping within the framework of international law and can take measures concerning pollution, safety and public order. The zone also forms an important component of national maritime security because it extends sovereign authority beyond the physical coastline. At the same time, the international right of innocent passage helps preserve the movement of vessels between different parts of the world. The balance between coastal sovereignty and international navigation is fundamental to the stability of maritime commerce. States therefore exercise territorial sea rights within a framework that combines national authority with international obligations.
The territorial sea is a maritime zone extending up to 12 nautical miles from a states applicable baseline in which the coastal state exercises sovereignty over the waters, airspace, seabed and subsoil, subject to international law and particularly the right of innocent passage. It should not be confused with the EEZ, which grants resource rights rather than full sovereignty, or with internal waters, which lie landward of the baseline.
Its boundaries are determined through technical geographic rules and, where necessary, agreements between neighbouring states. The territorial sea is one of the fundamental zones of modern maritime law because it defines the area in which coastal sovereignty extends from land into the ocean while preserving internationally recognised navigational rights.
