The Tapajós hydroelectric complex (Portuguese: Complexo Hidrelétrico de Tapajós) is a proposed complex of hydroelectric dams on the Tapajós and Jamanxim rivers in the state of Pará, Brazil. The Tapajós dams would contain locks, thus converting the river into a navigable waterway. A "platform" model is proposed under which all people and material would be moved by river or by helicopter, avoiding the need to build access roads and the consequent inflow of settlers and environmental damage. However, there have been protests against flooding of indigenous territory by the dams, and the largest dam seems unlikely to be approved.

The proposed Tapajós hydroelectric complex would impound sections of the Tapajós and Jamanxim rivers. The Tapajós River Hydroelectric Complex would have a total installed capacity of 10,682 MW. Eletronorte estimated that the project would deliver power equivalent to that provided by burning 30.5 million barrels of oil annually. The proposed dams being studied are:

Plant	Capacity	Guaranteed capacity	Dam Height (m)	Turbines	Type	Reservoir (km)	Flow (m/s)
São Luiz do Tapajós	8,040	4,012	39.0	38	Kaplan	729.00	11,890
Jatobá	2,338	1,282	35.5	40	Bulb	646.30	10,423
Jamanxim	881	475	72.0	3	Francis	74.45	1,366
Cachoeira do Cai	802	418	39.0	5	Kaplan	420.00	1,940
Cachoeira dos Patos	528	272	34.3	3	Kaplan	116.50	1,327

São Luiz do Tapajós would have guaranteed capacity of 4,012 MW from 38 Kaplan turbines. The dam on the Tapajós would be 7,608 metres (24,961 ft) long and 53 metres (174 ft) high and would impound a reservoir of 729 square kilometres (281 sq mi). There would be 17 locks, 19.6 metres (64 ft) wide and 20 metres (66 ft) high, in the spillway. The Jatobá Dam, also on the Tapajós, would be upstream from São Luiz do Tapajós. It would have 2,338 MW capacity, with 1,282 MW guaranteed, from 40 bulb turbines. The dam would be 1,287 metres (4,222 ft) long and 35.5 metres (116 ft) high impounding a reservoir of 646.3 square kilometres (249.5 sq mi). The 40 metres (130 ft) spillway would have 14 locks 18.9 metres (62 ft) wide and 20 metres (66 ft) high. The other three plants under study would be on the Jamanxim River.

The complex may also include the Chacorão (3,336 MW) on the upper Tapajós and the Jardim do Ouro (227 MW) on the Jamanxim. These have not yet been studied in detail.

The dams are part of a plan to convert the Tapajos into a waterway for barges to take soybeans from Mato Grosso to the Amazon River ports. A continuous chain of dams, with locks, would eliminate today's rapids and waterfalls. Legal and constitutional objections may be bypassed through "security suspensions." The Chacorão locks are listed as a priority in the National Waterways Plan (Brazil, MT 2010, p. 22). The dam's reservoir would eliminate the Chacorão rapids, allowing barge traffic above the dam's locks. The controversial Chacorão Dam is rarely discussed in the context of the Tapajós Basin developments, despite its central role in the plan.

The Tapajós projects are surrounded by conservation units with a total area of 200,000 square kilometres (77,000 sq mi), including the Amazônia National Park and the Itaituba I, Itaituba II and Jamanxim national forests. With the traditional construction model access roads are built to the plant, settlements form along the roads and around the plant, and there is widespread damage to the environment. This risk of damage to the native forest causes opposition to the projects.

The concept of building hydroelectric power plants in a similar way to offshore oil platforms was developed in 2004–05 planning sessions attended by Minister of the Environment Carlos Minc. With the platform model there would be minimal impact on the environment, with trees only cut down at the plant location, and the site later regenerated. All material and personnel would be transported by water or by helicopters. Transport costs will be higher but the need to build roads and other infrastructure will be avoided, so net costs may be lower.

The project would affect at least 32 communities and 2,000 square kilometres (770 sq mi) of indigenous territory, mostly occupied by people of the Munduruku ethnic group. The São Luiz do Tapajós Dam would flood about 7% of the Sawré Muybu Indigenous Territory, and the Boa Fé village would have to be relocated. As of 2010 Eletronorte had not applied for registration with the National Electricity Agency to start feasibility studies for the Chacorão plant, since it would flood parts of the Mundurucu and Sai-Cinza indigenous territories. A spokesman said that without a decree to regulate the constitution there was no way to undertake projects in indigenous territories.

On the Jamanxim River, the Jamanxim Dam would flood 8,516 hectares (21,040 acres) of the Jamanxim National Park. It would affect the South Amazon Ecotones Ecological Corridor. The Cachoeira do Cai reservoir would flood 15,690 hectares (38,800 acres) of the Jamanxim National Park, 6,800 hectares (17,000 acres) of the Itaituba I National Forest and 20,470 hectares (50,600 acres) of the Itaituba II National Forest. The official estimate is that 150 people will be affected. The Cachoeira dos Patos reservoir would flood 9,000 hectares (22,000 acres) of the Jamanxim National Park and 360 hectares (890 acres) of the Jamanxim National Forest. It would also affect the area around the Tapajós Environmental Protection Area and the South Amazon Ecotones Ecological Corridor.

An analysis of the two Tapajós River plants published in 2014 concluded that forecasts of demand for electricity had been overstated, and plans did not take into account improvements in efficiency of energy use and the availability of new power sources such as solar and wind power. Taking into account the cost of the transmission system they concluded that the project was not cost-effective even when excluding factors such as loss of fishing and tourism revenue, water quality degradation and carbon emissions.

A study released in December 2015 took into account carbon and methane emissions from construction and from the Cachoeira do Cai and Cachoeira dos Patos reservoirs, and concluded that there was a high probability that the plants would generate emissions comparable to a natural gas plant. In the case of the Cachoeira do Caí there was a possibility that emissions could exceed those of a coal-fired plant.

The first surveys of the hydroelectric potential of the Tapajós River were made between 1986 and 1991, and the planning process resumed early in the 2000s. In 2009 Eletronorte, Camargo Corrêa and CNEC Engenharia undertook hydraulic inventory studies of the Tapajós and Jamanxim Rivers, which were approved by the Brazilian Electricity Regulatory Agency (Aneel). By 2010 the Brazilian Institute of Environment and Renewable Natural Resources (IBAMA) had started the licensing process for five dams on the Tapajós with a total reservoir area of about 2,000 square kilometres (770 sq mi). These would all be platform-based plants. According to the Energy Expansion Plan 2010–19 the first plant, São Luiz do Tapajós, was to be launched in 2011 and to start operating in 2016.