The Baixada Fluminense (standard Portuguese pronunciation: [ˌbajˈʃadɐ flumiˈnẽjsi]; local pronounce: [ˌbɐ(j)ˈʃadɐ flumɪˈnẽ(j)si]) (literally "Fluminense Lowland") is a region in the Rio de Janeiro Metropolitan Region or Greater Rio (in the state of Rio de Janeiro), in southeastern Brazil. It is located on Guanabara Bay, between Rio de Janeiro to the south and the Serra dos Órgãos range of hills to the north.

It has an estimated population of 3,925,424, making it the second most populous region in the state, following only the city of Rio de Janeiro.

The expression "Baixada Fluminense" has two definitions: one in the stricto sensu sense (the predominant definition of the term in this article), and another in the lato sensu sense. In the strict sense, the expression refers to the political, economic, and social region discussed above, defined by its relationship with the rest of the metropolitan area of Rio de Janeiro, especially with the metropolitan core. In this sense, the term denotes the area of direct expansion of the Rio de Janeiro metropolis beyond the administrative limits of the core municipality. This definition of "Baixada Fluminense" is the most common today. Among the various delimitations of this region, the most comprehensive includes the municipalities of Guapimirim, Magé, Duque de Caxias, Nova Iguaçu, São João de Meriti, Nilópolis, Belford Roxo, Mesquita, Queimados, Japeri, Paracambi, Seropédica and Itaguaí (to which Mangaratiba is sometimes added, as this municipality is also part of the immediate urban expansion area of the Rio de Janeiro metropolis, along the Itaguaí axis).

In some official regional divisions of the state of Rio de Janeiro, certain municipalities considered part of the Baixada Fluminense in its broadest sense belong to other regions. For example, in the division of the state into tourist regions, Magé and Guapimirim are not included in the Baixada Fluminense but in the Serra Verde Imperial region; Itaguaí and Mangaratiba are in the Costa Verde region (Mangaratiba, specifically, has belonged since 2002—when it left the Metropolitan Region of Rio de Janeiro—to the Costa Verde government region in the state's political-administrative division—a region to which Itaguaí also belonged between 2002 and 2009—distinct from the Metropolitan Region); and Paracambi is in the Vale do Café region.

In a broader sense, the term refers to the physical geography of the state and denotes the entire plain region located between the Serra do Mar and the coast of Rio de Janeiro, from Mangaratiba to Campos dos Goytacazes. This region is further divided into four sectors: Baixada de Sepetiba, Baixada da Guanabara, Baixada de Araruama, and Baixada dos Goytacazes.

The expression "Baixada da Guanabara" is also sometimes used to refer to the Baixada Fluminense in its narrower sense—the portion of the metropolitan area of Rio de Janeiro formed by the 13 municipalities mentioned above—in order to distinguish this meaning of "Baixada Fluminense" from that referring to the region extending from the southern coast to the northern part of the state (Baixada Fluminense in the broad sense).

This region should not be confused with the Baixadas Litorâneas, located farther to the east in the same state.

The Baixada Fluminense, in the broad sense of the term, presents variable width: it is quite narrow in the initial stretch of the "Sepetiba Lowlands" (the section extending from the municipality of Mangaratiba to the Coroa Grande neighborhood, in Itaguaí, between the Serra do Mar and Sepetiba Bay), and gradually widens eastward until the Macacu River, in the "Guanabara Lowlands" region, which extends to the municipalities of Guapimirim and Cachoeiras de Macacu. In this stretch, within the municipality of Rio de Janeiro, between the West Zone (Sepetiba Lowlands), North Zone and South Zone (Guanabara Lowlands), rise the massifs of Pedra Branca Massif and Tijuca Massif, which reach altitudes slightly above one thousand meters. From Guanabara Bay to Cabo Frio, along the "Araruama Lowlands" sector, the Baixada Fluminense narrows again, presenting a succession of small elevations between 200 and 500 meters in height: the so-called "coastal massifs of Rio de Janeiro state". From the stretch between the district of Rocha Leão, in Rio das Ostras, and the neighborhood of Imboassica, in Macaé, the Baixada Fluminense widens once more in its final sector, the "Goytacazes Lowlands", reaching its greatest extent in the delta of the Paraíba do Sul River, in the region of Campos dos Goytacazes and São João da Barra, in northern Rio de Janeiro state.