Purpose: Long non-coding RNAs (lncRNAs) are cellular signaling molecules whose dysregulated expression is associated with malignancy. The Cancer Genome Atlas (TCGA) data shows that the novel lncRNA Psoriasis Susceptibility-Related RNA Gene Induced by Stress (PRINS) is underexpressed in breast cancer. The purpose of this study was to validate PRINS underexpression in breast cancer clinical samples and determine its clinical correlates, thus confirming PRINS as a breast cancer biomarker and providing insights into its cellular function. Methodology: 75 patient samples (25 normal breast, 25 primary breast tumours and 25 metastatic breast cancers) were obtained from The Kolling Institute Tumour Bank. Total RNA was extracted and relative PRINS expression was determined using RT-qPCR. De-identified clinicopathological data were collected retrospectively and the clinical correlates of PRINS expression were determined using subgroup univariate analysis. Results: Breast cancer primary tumours underexpressed PRINS relative to normal breast tissues (mean difference in log(2) PRINS expression 1.213; CI 95%, 0.389-2.038; p = 0.002). PRINS relative expression reliably discriminated between normal and malignant breast tissues (AUC, 0.851; CI 95%, 0.748-0.954; p <0.001; sensitivity, 80%; specificity, 72%). PRINS relative expression did not significantly differ between primary tumours and metastases. Subgroup univariate analysis revealed a significant association between advanced primary tumour histological grade and PRINS relative expression (F2,41 = 4.308; p = 0.021). Conclusion: PRINS is a biomarker for invasive breast cancer whose underexpression is associated with advanced histological grade.