Eco-efficiency could be defined as the simultaneous ability to achieve acceptable economic results with the least possible environmental degradation. Its analysis in crop and livestock production systems has become a hot topic among politicians and scientists. Pig pasture production systems are in high commercial demand because they are associated with high quality and environmentally friendly products. This work aimed to assess the eco-efficiency of pig farms and subsequently explore the determinants of inefficiency in the dehesa ecosystem in the southwest of the Iberian Peninsula. Farmers from 35 randomly selected farms were interviewed to obtain farm-level data. The eco-efficiency level was calculated through a joined data envelopment analysis (DEA) and life cycle assessment (LCA) approach. Subsequently, a truncated Tobit model was applied to determine factors associated with inefficiency. The results of the research revealed that Iberian pig farms are highly eco-efficient. The estimated average eco-efficiency score is 0.919 and ranges from 0.479 to 1, suggesting that the average farm could increase its value by about 8.1%. This means that the aggregate environmental pressures could be reduced by approximately this proportion (8%) while maintaining the same input level. The determinants related to social and demographic characteristics that positively affected eco-efficiency were the number of children, while years of farm activity and educational level had a negative effect. On the other hand, farm’s characteristics and the type of management, the percentage of own surface area, the percentage of livestock use, and the high proportion of pigs fattened in montanera, positively affected the eco-efficiency level.