The molecular mechanisms underlying testicular development in the yak remain poorly understood. In this study, RNA sequencing was performed on testicular tissues from Tianzhu white yaks at three postnatal developmental stages: 0.5 (prepubertal), 2.5 (pubertal), and 4.5 (adult) years, with six animals in each group. Differential expression analysis was integrated with weighted gene co-expression network analysis (WGCNA) to characterize transcriptomic changes associated with testicular maturation. A total of 9646 differentially expressed genes (DEGs) were identified between the 0.5- and 2.5-year groups, whereas only 126 DEGs were detected between the 2.5- and 4.5-year groups, indicating that the transition to puberty represents the most dynamic phase of transcriptional reprogramming during testicular development. Functional enrichment analysis revealed that focal adhesion and extracellular matrix–receptor interaction were the most significantly enriched Kyoto Encyclopedia of Genes and Genomes pathways, suggesting a potential role for extracellular matrix (ECM) remodeling in the establishment of the blood–testis barrier and testicular maturation. WGCNA identified seven age-associated co-expression modules. The turquoise module showed the strongest positive correlation with developmental stage and was significantly enriched for genes involved in spermatid differentiation and maturation, including SPATA6, SPMIP11, TNP1, and ODF1. Collectively, these findings provide a comprehensive transcriptomic characterization of yak testicular development.