<p>Operating multiple business models can be essential for firm survival, yet can also pose fundamental challenges for managers and organizations. Accordingly, business model diversification has emerged as an important phenomenon, attracting increasing attention from strategy and innovation scholars as well as practitioners. Despite this growing interest, research efforts remain dispersed, conceptually ambiguous, and theoretically underdeveloped, thereby impeding cumulative advancement. To address these gaps, we provide the first comprehensive systematic literature review of the business model diversification literature. Our review of 81 articles seeks to advance understanding of the concept and establish a foundation for future research and practice. We find that business model diversification encompasses four distinct processes, each associated with a unique set of activities, challenges, and objectives. We also identify divergent research streams, with one stream framing business model diversification as a strategy process and another as an innovation process. In addition, we highlight emerging trends related to sustainable and circular business models, digital transformation, and demand-side research. We further emphasize the central role of business model interrelatedness. This study contributes to current understanding by clarifying the conceptual landscape, offering a more nuanced interpretation of business model diversification, and situating it within the strategy and innovation literatures. Finally, we propose a novel framework for business model interrelatedness and outline important avenues for future research.</p>