Civilisational intergovernmental organisations occupy a distinctive position in global governance because their legitimacy is shaped by institutional performance, symbolic identity, collective representation, and normative expectation. This study develops a sentiment-driven computational modelling framework for analysing institutional perceptions in a 57-member civilisational intergovernmental organisation. The empirical corpus comprises 30 semi-structured interviews, including 15 expert interviews and 15 general respondent interviews. After preprocessing, removal of administrative material, and exclusion of courtesy-only responses, the final corpus contained 239 answer segments and 42,940 respondent-generated words. The methodology integrates answer-level segmentation, thematic classification, lexical sentiment analysis, transformer-based sentiment modelling, latent semantic clustering, non-parametric statistical testing, bootstrap confidence estimation, and manual validation. The results show that institutional perception is broadly positive but thematically uneven. Experts recorded a mean polarity score of 0.0803, while general respondents recorded 0.1016. However, group-level differences were not statistically significant at segment level, Mann–Whitney , , or interview level, , . In contrast, sentiment differed significantly across institutional themes, Kruskal–Wallis , , and across latent semantic clusters, with Kruskal–Wallis and . The strongest positive sentiment appeared in reform and future orientation, mean polarity , socioeconomic cooperation, , and institutional effectiveness, . The weakest sentiment concerned institutional weakness and constraints, , and political and security role, . Latent semantic clustering identified socioeconomic and economic cooperation as the most positively evaluated cluster, with mean polarity . Overall, the findings reveal a measurable tension between symbolic-developmental legitimacy and operational scepticism.