Territorial waters are informally an area of water where a sovereign state has jurisdiction, including internal waters, the territorial sea, the contiguous zone, the exclusive economic zone, and potentially the extended continental shelf (these components are sometimes collectively called the maritime zones). In a narrower sense, the term is often used as a synonym for the territorial sea.

Vessels have different rights and duties when passing through each area defined by the United Nations Convention on the Law of the Sea (UNCLOS), one of the most ratified treaties. States cannot exercise their jurisdiction in waters beyond the exclusive economic zone, which are known as the high seas.

Normally, the baseline is the low-water line along the coast as marked on large-scale charts that the coastal state recognizes. This is either the low-water mark closest to the shore or an unlimited distance from permanently exposed land, provided that some portion of elevations exposed at low tide but covered at high tide (such as mud flats) is within 3 nautical miles (5.6 kilometres; 3+1⁄2 statute miles) of permanently exposed land.

Straight baselines can alternatively be defined connecting fringing islands along a coast, across the mouths of rivers, or with certain restrictions across the mouths of bays. In this case, a bay is defined as "a well-marked indentation whose penetration is in such proportion to the width of its mouth as to contain land-locked waters and constitute more than a mere curvature of the coast. An indentation is not, however, regarded as a bay unless its area is as large as, or larger than, that of the semi-circle whose diameter is a line drawn across the mouth of that indentation". The baseline across the bay must also be no more than 24 nautical miles (44 kilometres; 28 statute miles) in length.

Internal waters are landward of the baseline. The coastal state has sovereignty over internal waters, and can enforce domestic law on vessels in internal waters, including to prohibit innocent passage. Lakes, rivers and bays are considered internal waters.

"Archipelagic waters" within the outermost islands of an archipelagic state, such as Indonesia or the Philippines, are also internal waters, but the state must allow innocent passage through them. However, archipelagic states can limit innocent passage to designated sea lanes within these waters. Each island in the archipelago can have its own baseline.

Territorial sea is a belt of sovereign water that extends at most 12 nautical miles (22 km; 14 mi) from the baseline of a coastal state, including the airspace over and seabed below it. Although the territorial sea is sovereign territory, foreign ships (military and civilian) are allowed innocent passage.

Boundaries can be adjusted through maritime delimitation. If the 12 nautical mile boundary of a state overlaps with the same boundary of another state, the border is taken as the median point between the states' baselines, unless they agree otherwise. A state can also choose to claim a smaller territorial sea.