The petrochemical industry plays a key role in Iran’s non-oil economy, yet it is also one of the major sources of greenhouse gas emissions. Assessing its environmental efficiency is strategically important for achieving the General Policies on the Environment, Energy, and the Resistance Economy, as well as for enhancing green competitiveness. This study evaluates the environmental efficiency and green competitiveness of ten major listed petrochemical companies in Iran during the period 2021–2025 using a non-oriented Slacks-Based Measure (SBM) model with undesirable outputs. Inputs include fixed assets, labor, and feedstock/energy costs, while the desirable output is operating revenue, and CO₂ emissions are treated as the undesirable output. The results show that the average environmental efficiency of the sample is 0.926, indicating a potential improvement of approximately 7.4%. Amir Kabir achieved full efficiency throughout the entire period and demonstrated the highest level of green competitiveness, whereas Jam and Nouri recorded the lowest scores. Inefficiency is mainly driven by excessive feedstock and energy consumption as well as higher CO₂ emissions. The findings can serve as a practical tool for monitoring the implementation of strategic policies and designing incentive mechanisms to improve the green performance of Iran’s petrochemical industry.