Urban trees, through the provision of diverse ecosystem services, play a significant role in improving the quality of life in megacities such as Tehran. Despite the importance of these services, the role of trees in enhancing Tehran’s air quality has received limited attention. In this study, using the i-Tree Eco model, the air quality improvement service—including the removal of air pollutants (CO, NO₂, SO₂, O₃, PM₁₀, PM₂.₅), carbon dioxide (CO₂) sequestration, and oxygen production—by Tehran’s urban trees in 2020 was estimated and economically valued using the Benefit Transfer method. The results showed that Tehran’s urban trees annually remove and sequester approximately 221,761.72 tons of pollutants and CO₂, with an estimated economic value of about USD 2,155,924 per year. In the same period, these trees also produced around 160,273 tons of oxygen; however, due to the abundance of oxygen in the atmosphere (about 21%), its economic value is considered negligible. The analysis of the ratio of pollutant and CO₂ removal by trees to their total emissions indicates their limited contribution to improving air quality. Therefore, relying solely on trees as a solution to air pollution and climate change is insufficient, and reducing emissions must be prioritized. These findings highlight the need to integrate nature-based solutions with complementary management and technological approaches. Moreover, urban trees provide other valuable ecosystem services, the quantification and valuation of which can offer a more comprehensive understanding of the real role of these green elements in urban resilience and sustainability.