Brazilian jurisdictional waters (Portuguese: águas jurisdicionais brasileiras, AJB) are the riverine and oceanic spaces over which Brazil exerts some degree of jurisdiction over activities, persons, installations and natural resources. They comprise internal waters, the territorial sea and exclusive economic zone (EEZ), to a distance of 200 nautical miles (370 kilometres) from baselines along the coast, as well as waters overlying the extended continental shelf, where Brazilian claims of jurisdiction are controversial, as the water column over this stretch of seabed is part of the high seas. The continental shelf of Brazil is under a different legal regime from its overlying waters. The Brazilian Navy covers both the shelf and the waters in its less formal concept of a "Blue Amazon".

The AJB's total claimed area stands at 5,669,852.41 km² (equivalent to 67% of land territory), of which 2,094,656.59 km² are above the extended shelf. These maritime zones are based on the United Nations Convention on the Law of the Sea (UNCLOS). From 1970 until it came into effect in 1994, Brazil had claimed a territorial sea as far as 200 nautical miles from the coast, instead of the present 12, but retains rights over natural resources in this area through its EEZ. Its coastline is the longest in the South Atlantic Ocean, but only three archipelagos contribute to its EEZ: Fernando de Noronha, Trindade and Martin Vaz and Saint Peter and Saint Paul.

Brazil's marine ecosystem is hydrographically and topographically complex and exhibits high rates of endemism and an economic potential in biotechnology. The two prevailing ocean currents, Brazil and North Brazil, have warm, nutrient-poor waters sustaining relatively low biomasses for each species, with a correspondingly limited fishing potential. In winter, cold waters of the Falkland Current may reach as far as the 24th parallel south and cold fronts and extratropical cyclones bring rough seas. The wind, waves, tides and thermal and osmotic gradients offer untouched potentials for renewable energy generation. 26.4% of the EEZ was under protected areas in 2021, mostly around the remote archipelagos of Saint Peter and Saint Paul and Trindade and Martin Vaz. Both are only populated by researchers and military personnel, which is one of the reasons for the government's marine science programs.

Most of the country's population lives near the coast and most of its international trade is conducted through the sea, but local shipbuilding and the national merchant marine have little presence in this trade. Coastal shipping answers a modest share of internal trade and mostly covers the oil and natural gas sector. There is no official measurement of the Brazilian maritime economy; 2015 estimates placed it at 2.67% of the gross domestic product directly tied to the sea, mostly in the tourism-dominated service sector. Coast guard duties in jurisdictional waters are assigned to the Navy.

Brazilian regulation on maritime spaces follows UNCLOS, a codification of international maritime law which came into force in 1994. This Convention, ratified by 168 states as of 2022, unifies centuries of rulemaking on interstate disputes over control of the seas. It organizes the sea off the coast of sovereign states in multiple zones: the territorial sea, contiguous zone, exclusive economic zone and continental shelf. Distances are measured in nautical miles from baselines along the coast. There are two kinds of baseline: normal lines, which follow the low-water line as plotted on official charts, or straight lines where the coast is too jagged or island-strewn.

The territorial sea extends up to 12 nautical miles (22 kilometres) from baselines and grants coastal states sovereignty over the airspace, water column, seabed and subsoil. Each of these spaces is treated separately in the other maritime zones. In the contiguous zone, at 12 to 24 miles from baselines, a coastal state does not have full sovereignty, but may take measures to prevent or repress unlawful activities in its territory or territorial sea. The contiguous zone is part of the EEZ, which has a width of 188 miles, from the limit of the territorial sea until a distance of 200 nmi (370 km) from baselines. This area gives a coastal state jurisdiction over the exploitation, conservation and management of its waters, seabed and subsoil. The high seas begin at the EEZ's outer limit.

The continental shelf as defined in international law is distinct from the geological continental shelf and consists in an area of seabed and its subsoil, excluding the overlying water column, over which a coastal state has sovereign rights over its natural resources. It extends "to the outer edge of the continental margin, or to a distance of 200 nautical miles from the baselines", as defined in the UNCLOS. If a coastal state's continental margin extends beyond 200 nmi, it may propose an extended continental shelf to the Commission on the Limits of the Continental Shelf (CLCS), an international organ created by the UNCLOS.

The term "Brazilian jurisdictional waters" (águas jurisdicionais brasileiras, AJB) exists in legislation since 1941 at the earliest, although it was less common than "Brazilian waters", "waters of the territorial sea" or "territorial waters". The territorial sea has had a legal definition since 1850, an exclusive fishing regime since 1938, a continental shelf since 1950, and a contiguous zone since 1966. An extended continental shelf was first proposed in 2004 and the country has yet to reach a full understanding with the CLCS to make its claims final and binding.