Background and Clinical Significance: This report presents the case of a 33-year-old female with recurrent miscarriage, investigated for an adrenal cortical adenoma characterized by autonomous secretion of 17-hydroxyprogesterone (17-OHP). The findings challenge the established diagnostic paradigm, which predominantly attributes elevated serum 17-OHP to congenital adrenal hyperplasia (CAH) or non-classical CAH (NCCAH). Case Presentation: The patient was found to have elevated serum 17-OHP and a 2 cm left adrenal mass. Normal testosterone and precursor levels, along with whole-exome sequencing (WES), argued against a diagnosis of non-classical 21-hydroxylase deficiency (NC-21OHD). An ACTH stimulation test elicited a mild-to-moderate rise in 17-OHP, while adrenal venous sampling (AVS) confirmed marked lateralization of 17-OHP hypersecretion to the left side. Postoperative normalization of 17-OHP levels further supported the diagnosis of a 17-OHP-secreting tumor. Histopathological analysis identified tumor regions with non-uniformly high expression of CYP17A1 and CYP21A2. Preliminary transcriptomic profiling revealed that differentially expressed genes (DEGs) were enriched in microRNA-related and PI3K-Akt signaling pathways. Conclusions: This paradigm-shifting case indicates that, in addition to 21OHD, a 17-OHP-hypersecreting adrenal adenoma should be considered in the differential diagnosis of elevated 17-OHP. The integration of multimodal diagnostic techniques, particularly AVS, is valuable for localizing hormonally active tumors. Preliminary mechanistic insights suggest a potential role for epigenetic dysregulation in the pathogenesis of this tumor type.