Out-of-pocket (OOP) health expenditure remains a serious welfare problem worldwide. The aim of this study was to investigate and decompose factors that are associated with inequalities in relative OOP health expenditure, estimated as the percentage of income spent on healthcare, in Saudi Arabia. Data from 10,785 respondents were obtained from a national cross-sectional survey conducted in Saudi Arabia as a part of the 2018 Family Health Survey. Inequalities in relative OOP health expenditure were measured using concentration indices and concentration curves. Moreover, the Wagstaff approach was used to decompose the concentration index of relative OOP health expenditure to assess the contribution of each of its determinants. The results revealed that relative OOP health expenditure in Saudi Arabia are concentrated among the poor (concentration index = −0.151, p < 0.01), resulting in a greater burden for the poor. Decomposition of the factors that contribute to this inequality revealed heterogeneity. Specifically, factors that increase the burden amongst the poor included aged above 60 years, and low levels of education, whereas factors that increase the burden amongst the rich included male gender, below the age of 60 years, secondary and higher education, having health insurance coverage, and suffering from chronic illnesses. Importantly, these results demonstrate that poor people might be exposed to hazardous health spending. Therefore, efforts to curbing OOP health expenditure should be framed by taking into account the specific factors that drive the burden towards the poor, such as older age and lack of education, so as to safeguard the overall welfare of the poor.