Online social comparison and self-concept have emerged as significant areas of research in the field of psychology, particularly in relation to the experiences of university students. This study assessed the relationship between the domains of online social comparison and self-concept of undergraduate university students in selected universities in Abia State, Nigeria. Based on the social comparison theory of Leon Festinger and self-concept theory of Carl Rogers, the study used an embedded mixed method design, with a target population of 15, 915 undergraduate university students. Simple random and snowball sampling procedures were used to select 274 participants for the quantitative strand, and 12 respondents for the qualitative strand respectively. The adapted Iowa-Netherlands Comparison Orientation Measure (INCOM) and the Robson Self-concept Scale (SCS) were used as quantitative instruments, while a semi-structured interview guide was used to collect qualitative data. Quantitative data was analyzed using descriptive and inferential statistics with the aid of the Statistical Package for Social Sciences (SPSS) version 25, while qualitative data was analyzed using thematic content analysis. The study showed a positive weak correlation between appearance social comparison and self-concept (r=0.233, p < 0.01); a positive weak correlation between opinion social comparison and self-concept (r= 0.132, p < 0.05) and a positive weak correlation between ability social comparison and self-concept (r=0.236, p < 0.01). Upward social comparison and self-concept were found to be negatively correlated (r=-0.131, p < 0.05) whereas, no significant correlation was found between downward social comparison and self-concept. The study therefore concludes that other factors may mediate the relationship between online social comparison and self-concept among university students.