ABSTRACT Objectives Of relevance to minority ethno-linguistic groups is the association between self-esteem and proficiency in their heritage language. The current study explored whether self-esteem of Cymraeg (Welsh) speaking pupils in Welsh/Bilingual-medium schools was higher than Cymraeg-speaking pupils in English-medium schools. Methods 1,709 pupils from Welsh/Bilingual and English-medium schools in Wales completed the Rosenberg Self-esteem Scale. Scores were analysed as a function of their primary language (language they use most often) and the linguistic medium of their school. Results Relative to primary-English speakers (pupils using English most) attending Welsh/Bilingual-medium schools, primary-Cymraeg speakers (pupils using Cymraeg most) attending Welsh/Bilingual-medium schools recorded higher self-esteem scores. There was no difference in self-esteem between speakers of different languages attending English-medium schools. Primary-English speakers attending English-medium schools recorded higher levels of self-esteem relative to primary-English speakers attending Welsh/Bilingual-medium schools. There was no difference in self-esteem between primary-Cymraeg speakers attending Welsh/Bilingual and English-medium schools. Longitudinal analysis suggests self-esteem increased for primary-Cymraeg speakers attending Welsh/Bilingual medium schools only, but pupils educated in their non-primary language demonstrated a decrease in self-esteem. Conclusion The results suggest care is needed when allocating pupils to schools, as language of the school and the pupil’s primary language interact in determining self-esteem over time.