In recent years, environmental uncertainty and market orientation have been considered key elements of superior firm performance. Although environmental uncertainty and market orientation may affect firm performance, innovation also mediates these effects. In this study, a conceptual model was developed to test the mediating effect of innovation on the relationships between these constructs. Data for the study were collected from Turkey's Top 500 Companies. Hierarchical regression and multiple regression analyses were employed to test the research hypotheses. The findings of the study revealed that the direct effects of environmental uncertainty and innovation on firm performance were statistically significant, although the effect of market orientation was not. The results obtained from the present study seem to indicate a possible "dual effect" of market orientation on firm performance. The results also indicated that innovation, especially product innovation, mediates the relationship between environmental uncertainty and firm performance. The findings indicate that innovation, especially product innovation, is critical for firm performance.