Collective marketing has long been regarded as a critical strategy to strengthen the bargaining power and boost smallholder farmers’ income. Collective marketing can reduce transaction costs and information asymmetries, allowing better access to output markets and increasing farm income. This study explores the determinants of the decision to participate in collective marketing and whether collective marketing increases farmers’ net farm income. The study utilized farm-level data of 351 calamansi farmers from Oriental Mindoro, the Philippines’ major calamansi producing province. An endogenous treatment regression model was used to analyze causality that links collective marketing to net farm income. The results of the analysis indicate that collective marketing positively and significantly influenced calamansi farmers’ net farm income. The decision to join collective marketing activities depends on farmers’ age, marketing cost, output price, access to credit, access to extension service, and distance to the nearest market. The endogenous treatment regression model estimates show that collective marketing resulted in higher net farm income compared to the counterfactual situation. The net farm income of calamansi farmers increased by PhP 21,619.40 per hectare due to collective marketing participation. Hence, collective marketing strategy could be used as a tool to boost the net farm income of calamansi farmers.