A common belief underlying the dimensional comparison theory is that reading self-concept has a positive relation to reading achievement but a negative relation to math achievement. However, this belief is incomplete and potentially misleading as supporting evidence is usually derived from studies without simultaneously considering the mediated effect of reading self-concept through reading achievement. The current study examined the direct and indirect effects of reading self-concept on math achievement through reading achievement, using PISA 2018 data from 501,766 students across 74 countries/territories. Multilevel structural equation modeling was performed to control for confounding variances from the school level and above. The results showed that reading self-concept had a direct and negative effect on math achievement, while also exerting a positive indirect effect on math achievement through reading achievement. Overall, the indirect effect outperformed the negative direct effect on math achievement. We concluded that reading self-concept exerted a double-edged effect on math achievement. These results should be able to shed new light on the contrasting comparison effect of self-concept on academic achievement.