The conventional assessment of project success has historically emphasized the triple constraint of cost, time, and scope, despite growing recognition that development projects should also be evaluated in terms of their social value, stakeholder outcomes, sustainability, and contribution to the Sustainable Development Goals (SDGs). This study examined the relationship between Social Impact Assessment (SIA) practices, SDG alignment, and project social success in development projects in Nigeria. Specifically, the study evaluated whether SIA planning quality, stakeholder engagement, and SIA monitoring and grievance mechanisms contribute to project social success directly and indirectly through alignment with selected SDGs. The SDG dimensions investigated were SDG 1 (No Poverty), SDG 3 (Good Health and Well-being), SDG 4 (Quality Education), SDG 8 (Decent Work and Economic Growth), and SDG 13 (Climate Action). Project social success was conceptualized in terms of overall social impact, community satisfaction, and long-term sustainability beyond project completion. A quantitative cross-sectional research design was adopted, using 221 project-level respondent observations involving project managers, monitoring and evaluation officers, and community liaison officers associated with development projects in Nigeria. Project type, funding source, and project duration were incorporated as controls. The study advances a project-success framework that separates delivery efficiency from development effectiveness and positions SDG alignment as a mechanism linking SIA practices with broader social outcomes. The framework provides a basis for evaluating development projects beyond cost, schedule, and scope and for institutionalizing SDG-linked social impact indicators within project appraisal, monitoring, stakeholder engagement, and post-completion sustainability assessment.