["Business Strategy and the Environment, EarlyView. ", "\nABSTRACT\nSmall and medium‐sized enterprises (SMEs) face growing pressure to integrate sustainable development in response to evolving environmental regulations, stakeholder expectations, and market demands. However, they often lack the financial, technological, and organizational resources and capabilities needed to successfully navigate their transitions to sustainable development. In this context, open innovation (OI) increasingly appears as a mechanism through which SMEs may access external knowledge, mobilize collaborative networks, and operationalize sustainability‐oriented transformations. Although prior research acknowledges the potential of OI to support sustainability, limited understanding exists regarding the organizational and relational mechanisms through which SMEs translate openness into environmental and social sustainability outcomes. This study addresses this gap by analyzing seven French SMEs across diverse sectors through a multiple‐case study approach. We investigate the motivations, mechanisms, and outcomes of OI adoption for sustainability. The findings reveal that sustainable open innovation (SOI) in SMEs is structured around four recurring motivational patterns: growth‐oriented, customer‐oriented, research‐oriented, and social sustainability‐oriented, each associated with distinct collaborative configurations and sustainability trajectories. The study shows that these patterns rely on different mechanisms, including strategic alliances, co‐creation practices, research collaborations, and stakeholder networks, which enable SMEs to access external knowledge, strengthen relational capital, develop eco‐designed products, support circular economy practices, and reinforce social inclusion initiatives. This research advances the SOI literature by showing how distinct collaborative configurations enable SMEs to convert openness into different sustainability trajectories. The findings provide guidance for managers and policymakers seeking to foster SME‐oriented sustainable innovation ecosystems.\n"]