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.