Purpose: Past research shows the Big-Fish-Little-Pond Effect (BFLPE; negative effect of school-average achievement on student-level academic self-concept) to generalize across countries. However, such evidence is largely limited to math and science. Given that reading self-concept is highly differentiated from math and science self-concepts and plays an important role in predicting educational outcomes, it is essential to examine the universality of the BFLPE and its underlying social-comparison process within this domain. Method: We assess the cross-national generalizability of the BFLPE for 15-year-olds' reading self-concept using Programme for International Students Assessment 2018 (533,165 students, 72 countries). To demonstrate that the BFLPE operates with a relative--rather than absolute--frame of reference for comparison, we juxtapose difficulty experienced with reading in general (self-concept perceived difficulty; relative frame of reference), with difficulty experienced with reading specifically during the PISA test (PISA test difficulty; absolute frame of reference). Results: Our findings show that the BFLPE for both the reading self-concept perceived competence and difficulty subscales was robust across countries. Further, the BFLPE was strong for self-concept subscales, but very weak for the PISA test difficulty scale. Conclusions: Our findings extend support for the generalizability of the BFLPE to reading self-concept and highlight the role of social comparison processes underlying this effect.