The World Health Organization recommends exclusive breastfeeding (EBF) for 6 months and continued breastfeeding for at least 2 years. Social support has been widely recognized to influence breastfeeding practices. However, existing scales do not measure exclusive breastfeeding social support (EBFSS), rather they assess social support for any breastfeeding. Further, they are tailored towards high-income settings. Therefore, our objectives were to develop and validate a tool to measure EBFSS in low-income settings. To develop the scale, local and international breastfeeding experts were consulted on modifications to the Hughes' Breastfeeding Social Support Scale. It was then implemented in an observational cohort in Gulu, Uganda, at 1 (n = 238) and 3 (n = 237) months post-partum (NCT02925429). We performed polychoric and polyserial correlations to remove redundant items and exploratory factor analysis at 1 month post-partum to determine the latent factor structure of EBFSS. We further applied confirmatory factor analysis to assess dimensionality of the scale at 3 months post-partum. We then conducted tests of predictive, convergent, and discriminant validity against EBF, self-efficacy, general social support, and depression. The modification of the Hughes' scale resulted in 18 items, which were reduced to 16 after examining variances and factor loadings. Three dimensions of support emerged: Instrumental, Emotional, and Informational, with alpha coefficients of 0.79, 0.85, and 0.83, respectively. Predictive, convergent, and discriminant validity of the resultant EBFSS scale was supported. The EBFSS scale is valid and reliable for measuring EBFSS in northern Uganda and may be of use in other low-income settings to assess determinants of EBF.