Testicular germ-cell tumours (TGCTs) are the most common cancer in young men; the incidence is increasing worldwide and they have an unusually high rate of metastasis. Despite significant work on TGCTs and their metastases in humans, absence of a mouse model of spontaneous metastasis has greatly limited our understanding of the mechanisms by which metastatic potential is acquired and on their modes of dissemination. We report a new model of spontaneous TGCT metastasis in the 129 family of mice and provide evidence that these are true metastases derived directly from primary testicular cancers rather than independently from ectopic stem cells. These putative metastases (pMETs) occur at similar frequencies among TGCT-affected males in six genetically distinct TGCT-susceptible strains and were largely found in anatomical sites that are consistent with patterns of TGCT metastasis in humans. Various lines of evidence support their pluripotency and germ-cell origin, including presence of multiple endodermal, mesodermal and ectodermal derivatives as well as cells showing OCT4 and SSEA-1 pluripotency markers. In addition, pMETs were never found in males that did not have a TGCT, suggesting that metastases are derived from primary tumours. Finally, pMETS and primary TGCTs shared several DNA copy number variants suggesting a common cellular and developmental origin. Together, these results provide the first evidence for spontaneous TGCT metastasis in mice and show that these metastases originate from primary TGCTs rather than independently from ectopic stem cells.