The territorial sea is a belt of ocean extending up to 12 nautical miles (about 22 kilometers) from a country’s coastline, over which that country exercises full sovereignty. Think of it as an extension of the nation’s territory into the water. The coastal state controls not just the surface but also the airspace above, the seabed below, and everything underneath the seabed. This zone and the rules governing it are established by the United Nations Convention on the Law of the Sea (UNCLOS), the international treaty that serves as the legal framework for the world’s oceans.
How the 12-Mile Limit Works
The territorial sea is measured outward from a country’s “baseline,” which is normally the low-water line along the coast as marked on official nautical charts. From that line, the state can claim up to 12 nautical miles of ocean as its territorial sea. Everything landward of the baseline, such as harbors, bays, and rivers, is classified as “internal waters,” which carry even stronger sovereign protections.
Not every coastline is a smooth, predictable line. Where a coast is deeply indented, cut into by fjords, or fringed by a chain of nearby islands, a country may draw “straight baselines” connecting points along the coast or outer islands. These straight lines then become the starting point for measuring the 12 miles. International law sets limits on this: the lines can’t depart significantly from the general direction of the coast, no single segment should exceed 48 nautical miles, and the enclosed waters must be closely linked to the land. These rules exist to prevent countries from drawing wildly expansive baselines to grab extra ocean.
Origins of the Rule
For centuries, the concept of territorial waters was tied to a practical measure: the range of a cannon shot from shore. If a nation could defend a stretch of water with its coastal guns, that water was considered its territory. By the 18th century, this evolved into a three-mile limit, roughly the maximum distance a cannonball could travel. Many nations held to the three-mile rule for over two hundred years. The modern 12-nautical-mile standard was formalized when UNCLOS was adopted in 1982, reflecting both advances in technology and the desire of coastal states for greater maritime control.
What a Country Can Do in Its Territorial Sea
Within the territorial sea, a coastal state has nearly the same authority it holds on land. It can enforce its criminal and civil laws, regulate fishing, control pollution, manage resources on the seabed, and restrict unauthorized activities. Foreign aircraft have no automatic right to fly through the airspace above the territorial sea without permission.
A state can also take enforcement action against foreign merchant ships in certain situations. If a ship leaves a country’s internal waters (like a port) and is still transiting the territorial sea, the coastal state can stop and inspect it. Ships heading into port can be checked for compliance with the conditions of entry. This level of authority makes the territorial sea fundamentally different from zones further offshore, where a country’s power is much more limited.
The Right of Innocent Passage
The one major limit on a country’s sovereignty in its territorial sea is a concept called innocent passage. Foreign ships have the right to travel through the territorial sea without needing permission, as long as their passage is “not prejudicial to the peace, good order, or security of the coastal State.” In practice, this means a cargo ship or fishing vessel can transit through without stopping, and the coastal state generally can’t block it.
Passage stops being “innocent” if the ship does things like conduct weapons exercises, gather intelligence, launch aircraft, load or unload goods in violation of customs laws, cause serious pollution, fish without authorization, or engage in any activity that threatens the coastal state’s security. If a ship crosses that line, the coastal state can take action to stop it, board it, or redirect it.
Countries can also temporarily suspend innocent passage in specific areas of their territorial sea when essential for security, such as during military exercises. The suspension must be officially published in advance and cannot discriminate between foreign ships of different nationalities.
How It Differs From Other Maritime Zones
The territorial sea is the zone where a coastal state holds the most power. Beyond it, authority diminishes in layers.
- Contiguous zone (up to 24 nautical miles from the baseline): The state can enforce customs, immigration, tax, and sanitation laws here, but only on the water’s surface and seabed. There are no airspace rights, and it functions more as a buffer zone than sovereign territory.
- Exclusive economic zone (up to 200 nautical miles): The state has rights over natural resources, both living (like fish) and nonliving (like oil and gas), and can regulate energy production, scientific research, and environmental protection. But it cannot restrict freedom of navigation or overflight. Other nations’ ships and planes can pass through freely.
The key distinction is sovereignty versus resource rights. In the territorial sea, the state is sovereign, similar to being on its own soil. In the EEZ, the state has economic rights but doesn’t control passage. In the contiguous zone, the state has limited enforcement powers focused on specific categories of law. The further from shore, the less control the coastal state exercises, and the more freedom other nations enjoy.
Internal Waters vs. Territorial Sea
Waters on the landward side of the baseline, such as ports, harbors, and enclosed bays, are classified as internal waters. A country has absolute sovereignty there, with no obligation to grant innocent passage to foreign ships. A state can apply all its domestic laws to any foreign merchant vessel in its ports and internal waters, just as it would on land. The territorial sea, by contrast, requires the coastal state to allow innocent passage. That distinction matters in practice: a country can refuse entry to its ports but generally cannot block a foreign ship from transiting peacefully through its territorial sea.