Brazil is currently witnessing the dawn of its offshore wind industry, and companies, government, investors, and society must understand the risks and possible environmental impacts this technology can generate. This paper aims to partially fill this need by presenting an analysis of the environmental impacts that would be caused by a 5 MW floating offshore wind turbine to be installed on the Brazilian continental shelf through an Environmental Impact Assessment (EIA) and a Life Cycle Assessment (LCA). We assumed that the wind turbine would supply electrical power to a floating oil and gas extraction platform, with the intention of reducing the amount of energy produced with fossil fuels in these platforms, in order to decrease the carbon footprint of this economic activity. The turbine would be mounted on a semi-submersible platform with a high mass of steel, and a battery system for energy storage. We considered two different sites for the turbine installation, Campos Basin and Santos Basin, which are the most important areas of oil and gas extraction in Brazil. The EIA examines the effects caused by the turbine in the ecosystems around it, showing that the fauna suffers from various impacts such as sedimentation, electromagnetic fields, and others, but few species are seriously affected, except for birds, which can have a risk of mortality. The LCA makes an assessment on the carbon dioxide (CO2) emissions and energy consumption for each part of the life cycle of the project, finding a total 21.61 g of CO2 emitted per kWh of energy produced by the turbine. The total energy consumed was 89,131.31 GJ, which causes an Energy Payback Ratio (EPR) of 16.28 and Energy Payback Time (EPT) of 1.23 years. Several sensitivity analyses were performed to understand the effect of the variation of several parameters related to recycling, maintenance and failures, and the capacity factor, on the values of CO2 emission and energy consumption. These analyses showed that variations in the amount of steel recycled and in the capacity factor of the system cause the most significant changes in EPR and EPT.