Shun Bai, Kaiqiang Fu, Huiqi Yin, Yiqiang Cui, Qiuling Yue, Wenbo Li, Le Cheng, Huanhuan Tan, Xiaofei Liu, Yueshuai Guo, Yingwen Zhang, Jie Xie, Wenxiu He, Yuanyuan Wang, Hua Feng, Changpeng Xin, Jinwen Zhang, Mingyan Lin, Bin Shen, Zheng Sun, Xuejiang Guo, Ke Zheng, and Lan Ye Transcription factors of the Sox protein family contain a DNA-binding HMG box and are key regulators of progenitor cell fate. Here, we report that expression of Sox30 is restricted to meiotic spermatocytes and postmeiotic haploids. Sox30 mutant males are sterile owing to spermiogenic arrest at the early round spermatid stage. Specifically, in the absence of Sox30, proacrosomic vesicles fail to form a single acrosomal organelle, and spermatids arrest at step 2-3. Although most Sox30 mutant spermatocytes progress through meiosis, accumulation of diplotene spermatocytes indicates a delayed or impaired transition from meiotic to postmeiotic stages. Transcriptome analysis of isolated stage-specific spermatogenic cells reveals that Sox30 controls a core postmeiotic gene expression program that initiates as early as the late meiotic cell stage. ChIP-seq analysis shows that Sox30 binds to specific DNA sequences in mouse testes, and its genomic occupancy correlates positively with expression of many postmeiotic genes including Tnp1 , Hils1 , Ccdc54 and Tsks . These results define Sox30 as a crucial transcription factor that controls the transition from a late meiotic to a postmeiotic gene expression program and subsequent round spermatid development.