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Abstract Sex chromosomes play an outsized role in adaptation and speciation, and thus deserve particular attention in evolutionary genomics. In particular, fusions between sex chromosomes and autosomes can produce neo‐sex chromosomes, which offer important insights into the evolutionary dynamics of sex chromosomes. Here, we investigate the evolutionary origin of the previously reportedDanausneo‐sex chromosome within the tribe Danaini. We assembled and annotated genomes ofTirumala septentrionis(subtribe Danaina),Ideopsis similis(Amaurina),Idea leuconoe(Euploeina) andLycorea halia(Itunina) and identified their Z‐linked scaffolds. We found that theDanausneo‐sex chromosome resulting from the fusion between a Z chromosome and an autosome corresponding to theMelitaea cinxiachromosome (McChr) 21 arose in a common ancestor of Danaina, Amaurina and Euploina. We also identified two additional fusions as the W chromosome further fused with the synteny block McChr31 inI. similisand independent fusion occurred between ancestral Z chromosome and McChr12 inL. halia. We further tested a possible role of sexually antagonistic selection in sex chromosome turnover by analysing the genomic distribution of sex‐biased genes inI. leuconoeandL. halia. The autosomes corresponding to McChr21 and McChr31 involved in the fusions are significantly enriched in female‐ and male‐biased genes, respectively, which could have hypothetically facilitated fixation of the neo‐sex chromosomes. This suggests a role of sexual antagonism in sex chromosome turnover in Lepidoptera. The neo‐Z chromosomes of bothI. leuconoeandL. haliaappear fully compensated in somatic tissues, but the extent of dosage compensation for the ancestral Z varies across tissues and species.