This study develops an SDG-aligned social impact assessment (SIA) system for five Chulalongkorn University research projects, in Thailand. Employing mixed methods in an exploratory sequential design, the first phase involves qualitative data collection, including focus groups and field studies, to establish the SIA system. The second phase employs quantitative data collection and applies SIA and social return on investment (SROI) approaches to evaluate these projects. The study yields the 'triple S system' (3S), comprising three components: strategic SDGs (S1); social impact assessment models (S2) encompassing goal-based, expert, and development evaluations; and SIA steps (S3), including the six-step '3I3S process'. These steps involve SDG indicator mapping (I1), impact pathway analysis (I2), identification of output, outcome, and impact indicators (I3), site visits and focus groups (S1), SROI analysis (S2), and summarisation of SDG social impact assessment results (S3). The study reveals alignment with specific SDGs (2, 4, 5, 8, 11, and 17) across the five projects. Projected 5-year SROI assessments range from 1.25 to 9.82. This dynamic system effectively monitors project progress toward social impact while identifying opportunities for enhancing the SIA process. This study contributes an SDG-aligned SIA system to enhance research project impact assessment, fostering more effective, sustainable, and socially beneficial initiatives.