Energy retrofitting of unsustainable apartment buildings will play a key role in achieving net zero targets globally, including in Saudi Arabia. While the energy and economic impacts are well studied in literature, the environmental impacts of energy retrofitting apartment buildings remain underexplored. This paper addresses this critical gap and evaluates the environmental performance of a residential apartment building in Saudi Arabia using an integrated approach that combines energy simulation with Life Cycle Assessment (LCA). The LCA was conducted following a cradle-to-gate plus use-phase system boundary and the functional unit was defined as the apartment building over an assumed lifespan of 60 years. The energy model was developed in Autodesk Revit and simulated in EnergyPlus to assess the impact of several Energy Efficiency Measures (EEMs). Environmental impacts were quantified using the ReCiPe 2016 Midpoint method across 18 categories. Results show that the retrofitted apartment building achieved a 25.5% reduction in annual energy consumption, with the Energy Use Index (EUI) decreasing from 180.53 to 128.4 kWh/m2/year. The LCA results showed notable reductions in all environmental categories, including Global Warming Potential (GWP), ozone depletion, and fossil resource scarcity. The total GWP decreased from 5.34 & times; 106 kg CO2 to 3.84 & times; 106 kg CO2, despite a slight increase in embodied emissions due to added materials such as insulation. By integrating energy modelling with LCA, the study provides environmental impact insights for residential retrofitting within the Saudi context and supports policymakers, designers, and stakeholders in incorporating carbon impacts into retrofit decision-making.