Although knowledge integration has been deemed relevant for the development and survival of family firms, prior studies suggest that the lack of skills to combine and transfer incumbent family members' knowledge within the family may hinder family firms' organizational effectiveness. Knowledge integration, in this context, might depend on family involvement in ownership, management and governance, and family essence, considering the emotional and relational aspects of the influence the family exerts on the business. Relying on knowledge-based view and dynamic capability perspectives, we propose a model suggesting that knowledge integration intervenes in the relationship between familiness and organizational effectiveness. Our hypotheses are tested on a sample of 102 private Spanish family businesses. The results reveal that familiness is beneficial for knowledge integration, enhancing family firms' organizational effectiveness. Our study provides practical implications for family business owners and managers, and opportunities for further research.