Windows are crucial in regulating indoor daylighting, cooling, and heating to ensure suitable living conditions. However, conventional windows lack the capability for spectrum splitting and exhibit unpredictable light and heat management. This study introduces a smart window to propose a reversible function by utilizing Antimony Tin Oxide (ATO) nanofluids with near-infrared (NIR) selective absorption. An experimental study was conducted in a controlled chamber to evaluate these smart windows' dynamic light and thermal environment. The study examined the effects of ATO nanofluid concentrations at 10 ppm, 100 ppm, and 500 ppm. Results showed that the maximum indoor air temperature decreased by 4.35 °C, 2.78 °C, and 4.29 °C, respectively. Additionally, the maximum temperature differences on the outer surfaces of the exterior glass (EG) and the control glass (CG) were 4.27 °C, 3.78 °C, and 4.43 °C, respectively. When the smart window was oriented to the east, west, or north, the maximum indoor temperature differences between EG and CG were 1.37 °C, 2.9 °C, and 2.43 °C, respectively. This study demonstrates the potential of ATO nanofluid-based smart windows to enhance indoor environmental control through effective light and heat management.