Germ cell fate is tightly controlled to preserve lineage fidelity, yet the mechanisms limiting developmental plasticity remain incompletely understood. Here, we identify the RNA-binding protein Dead end1 (Dnd) as a central regulator that constrains fate potential in primordial germ cells (PGCs) by modulating Nanog expression and associated pluripotency programs. Loss of Dnd disrupts Nanog mRNA localization, triggers aberrant activation of somatic lineage genes, and drives transdifferentiation of PGCs into mesodermal and neuroectodermal fates. Stimulating proliferation through Cyclin D1 partially rescues germ cell identity in Dnd-deficient PGCs by restoring Nanog mRNA localization and transcriptional regulation. Conversely, ectopic co-expression of Dnd and Nanos can reprogram mesodermal somatic cells toward a germ cell-like state, but this process fails in Nanog-depleted embryos, demonstrating that Nanog is indispensable for permissive fate conversion. Together, these findings establish Dnd as both a safeguard and instructor of germline identity, acting through control of Nanog and reinforcement of fate decisions.