Introduction: The Multidimensional Scale of Perceived Social Support (MSPSS) is a tool widely used in health surveys to assess social support (SS) perceived by individuals. The lack of an appropriate tool to measure SS perceived by patients with cancer has hindered our understanding of how SS is associated with the management and recovery process of Sri Lankan patients with cancer. The study aimed to examine the reliability and validity of the Sinhalese version of the MSPSS.Methods: Cross-cultural adaptation of the MSPSS was done following the standard guidelines of the process of translation and adaptation set by the World Health Organization. The study sample consisted of 40 patients with cancer at the Teaching Hospital Karapitiya, Galle, Sri Lanka. Data analysis was done using SPSS 25.0. Internal consistency of the overall MSPSS was calculated using Cronbach’s alpha. Test-retest reliability of the tool was also assessed using the intra-class correlation coefficient (ICC). The convergent and divergent validity of the tool was assessed using the Centre for Epidemiological Studies-Depression (CES-D) and the World Health Organization-Quality of Life-Brief Scale (WHOQoL-BREF). Construct validity was assessed using Exploratory Factor Analysis (EFA).Results: The Sinhalese version of MSPSS showed high reliability (Cronbach’s alpha-0.911). The tool showed favourable test-retest reliability (ICC=0.91). The overall MSPSS score was correlated negatively with depressive symptoms (r= -0.459, p<0.001) and positively with dimensions of quality-of-life scores; overall (r=0.674, p<0.001), physical (r=0.426, p<0.001), psychological (r=0.540, p<0.001), social (r=0.674, p<0.001) and environmental quality of life (r=0.767, p<0.001) ensuring strong discriminant/divergent and convergent validity of the Sinhalese version of the MSPSS. Factor analysis extracted three factors (e.g., family, friends, and significant others) explaining 96.65% of the variance.Conclusion: The Sinhalese version of MSPSS is a reliable and valid tool to assess the perceived SS from family, friends, or significant others among patients with cancer.