Service innovation is a key element in firms’ performance. However, most studies have paid particular attention to the analysis of business’ internal resources and their effects on innovation, regardless to the role of some other relevant characteristics such as external environment of firms. Using a sample of 108 Tunisian service firms, the goal of this paper is to analyze the relationship between internationalization and innovation which represents a central issue in the international business literature. Our results confirm that internationalization enhances a firm’s innovation capacity. This result suggests that access to external knowledge clearly explains the innovation performance of Tunisian firms.