The rapid increase in municipal solid waste in developing areas, as demonstrated by the Fez landfill, has led to the production of leachate with alarmingly high levels of pollutants, highlighting the urgent need for comprehensive analysis and advanced treatment methods. This study uncovers exceptionally high concentrations of organic and inorganic substances, and heavy metals in the leachate, with Chemical Oxygen Demand (COD) reaching 57,100 mg/L, Biochemical Oxygen Demand (BOD₅) at 39,400 mg/L, and a significant presence of iron (Fe) at 1,370 mg/L, underscoring the extreme contamination levels. Importantly, this research introduces a notable contribution to the field by calculating a Leachate Pollution Index (LPI) of 88.8, a figure considerably exceeding the limits considered safe for environmental discharge. The results of the current study complement already existing information about the critical environmental threat posed by the leachate, emphasizing the necessity for extensive treatment prior to release into the environment. The study's insights are crucial for developing effective strategies to reduce the risks to aquatic ecosystems and public health, and for guiding policy and practice in waste management in developing regions.