This paper explores deep energy retrofits for office buildings in El Salvador, aiming for net-zero energy performance while keeping acceptable indoor thermal comfort conditions for occupants. While much research focuses on new construction, retrofitting older buildings is crucial for sustainability, especially in tropical climates. Using energy modeling and optimization, the study defines a baseline office building and applies both passive and active energy-saving measures. Key strategies include improving ventilation, lighting, and HVAC systems and enhancing the building envelope through glazing and shading upgrades. These interventions reduced the building’s energy consumption from 285.41 kWh/m2 to 58.87 kWh/m2 annually. A photovoltaic system was added to achieve a net-zero energy balance. The study provides actionable insights for sustainable retrofitting, demonstrating the potential for significant energy savings while maintaining indoor comfort in tropical climates.