Integrating the resource-based theory, the relational view, and the contingency theory, this research advances the strategic alliance literature by providing a theoretical framework that explains alliance outcomes from both the inter-organizational and the external environmental perspectives. Specifically, we analyzed the effect of complementary resources on alliance performance through the mechanism of relational capital in an uncertain environment. We also explored the moderating roles of environmental dynamism and environmental hostility in the indirect relationship between resource complementarity and alliance performance with a moderated mediation model. Based on the empirical evidence from a survey of 210 alliance firms, we found that complementary resources that a firm can access from its strategic alliance motivate it to invest relational capital in the partnership, which in turn improves alliance performance. However, the positive link between resource complementarity and relational capital is attenuated under a highly dynamic environment. More importantly, results of the moderated mediation analysis suggest that the mediating effect of relational capital between resource complementarity and alliance performance is stronger when the environment is less dynamic, but this effect is not moderated by environmental hostility. These findings imply that complementary resources are critical antecedents of alliance performance, yet firms could not leverage the value of complementary resources to achieve alliance success without sound relational mechanisms or the ability to adapt to the uncertain environment.