While several studies have recently examined horizontal mismatch among different individuals in the labour market, no empirical evidence from Saudi Arabia has emerged on this increasingly relevant issue in the ongoing debates about education outputs and labour market requirements. This study aimed to extend the literature by investigating the incidence and individual determinants of horizontal mismatch among Saudi tertiary graduates, using cross-sectional data based on two worker self-assessment methods. The results revealed that the observed incidence of mismatch differed within each measure depending on the level of categorisation used to classify graduates as matched or mismatched. Less than one-sixth of the graduates in the study were mismatched on each measure when using a detailed categorisation, while more than one-third were mismatched on each measure based on a dichotomous categorisation. Furthermore, the study provided evidence of the importance of several individual characteristics in explaining the probability of horizontal mismatch among graduates. Irrespective of the measure used, education level, field of study, type of contract, and work experience were all found to be major determinants of the probability of being mismatched among Saudi graduates.