Human fetal germ cells (FGCs) are precursors to sperm and eggs and are crucial for maintenance of the species. However, the developmental trajectories and heterogeneity of human FGCs remain largely unknown. Here, we performed single-cell RNA-seq analysis of over 2000 FGCs and their gonadal niche cells in female and male human embryos spanning several developmental stages. We found that the female FGCs undergo four distinct sequential phases characterized by mitosis, retinoic acid signaling, meiotic prophase, and oogenesis. Male FGCs develop through stages of migration, mitosis and cell cycle arrest. Individual embryos of both sexes simultaneously contain several subpopulations, highlighting the asynchronous and heterogeneous nature of FGC development. Moreover, we observed reciprocal signaling interactions between FGCs and their gonadal niche cells, including activation of BMP and Notch signaling pathways. Our work provides key insights into crucial features of human FGCs during their highly ordered mitotic, meiotic, and gametogenetic processes in vivo.