Human activities have transformed nearly three-quarters of the Earth's land cover, with urbanization being one of the most irreversible forms of land use change. Large infrastructure projects often accelerate these transformations, reshaping landscapes and ecosystems. This study examines the spatial and urban transformations linked to the Rio de Janeiro Petrochemical Complex (COMPERJ) in Eastern Metropolitan Rio de Janeiro between 2006 and 2023. Using remote sensing data (MapBiomas Collection 9) and census data from the Brazilian Institute of Geography and Statistics (IBGE), we assess how the construction—and subsequent halt—of COMPERJ influenced land use and land cover (LULC) changes, urban expansion, and population dynamics. Results reveal accelerated urban growth (19.51%) and population growth (13.94%) between 2000 and 2010, driven by expectations of economic development, followed by population decline (-0.74%) after the project's suspension, while urban areas continued expanding (5.49%) at lower rates, at the expense of forest and productive areas. The findings demonstrate that large infrastructure projects can trigger irreversible urbanization, even when economic promises fail, leading to inefficient land use and reduced urban densities.