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R‐spondins (RSPOs) are influential signaling molecules that promote the Wnt/β‐catenin pathway and self‐renewal of stem cells. Currently, RSPOs are emerging as clinically relevant oncogenes, being linked to cancer development in multiple organs. Although this has instigated the rapid development and testing of therapeutic antibodies targeting RSPOs, functional evidence that RSPO causally drives cancer has focused primarily on the intestinal tract. Here, we assess the oncogenic capacity of RSPO in breast cancer in a direct fashion by generating and characterizing a novel mouse model with conditional Rspo3 expression in the mammary gland. We also address the prevalence of RSPO gene alterations in breast cancer patients. We found that a quarter of breast cancer patients harbor RSPO2/RSPO3 copy number amplifications, which are associated with lack of steroid hormone receptor expression and reduced patient survival. Foremost, we demonstrate the causal oncogenic capacity of RSPO3 in the breast, as conditional Rspo3 overexpression consistently drives the development of mammary adenocarcinomas in our novel Rspo3 breast cancer model. RSPO3‐driven mammary tumors typically show poor differentiation, areas of epithelial‐to‐mesenchymal transition, and metastatic potential. Given the reported interplay in the Wnt/β‐catenin pathway, we comparatively analyzed RSPO3‐driven mouse mammary tumors versus classical WNT1‐driven analogues. This revealed that RSPO3‐driven tumors are distinct, as the poorly differentiated tumor morphology and metastatic potential were observed in RSPO3‐driven tumorigenesis exclusively, further substantiated by differentiating gene expression profiles. Co‐expression of Rspo3 and Wnt1 transduced mammary tumors with a mixed phenotype harboring morphological features characteristic of both transgenes. In summary, we report that a quarter of breast cancer patients harbor RSPO2/RSPO3 copy number gains, and these patients have a worse prognosis, whilst providing in vivo evidence that RSPO3 drives poorly differentiated invasive breast cancer in mice. Herewith, we establish RSPO3 as a driver of breast cancer with clinical relevance, proposing RSPO3 as a novel candidate target for therapy in breast cancer. © 2022 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland. [ABSTRACT FROM AUTHOR]