Environmental sustainability can require extensive investment from suppliers as well as firms. Previous studies on this topic focused on the direct relationship between supplier collaboration and environmental performance, yet these studies did not investigate the mechanism behind the relationship. Our study examines the mediating role of innovation on the relationship between supplier collaboration and environmental performance. We tested this model using a sample of 597 Korean firms and found that innovation positively mediated the collaboration effect with suppliers through measurements of strategic, operational, and product development collaboration. These results help us identify the process through which supplier collaboration leads to better environmental performance.