Prostate cancer (PCa) is a major cause of cancer-related mortality in men worldwide. PCa mortality is expected to almost double by 2040 which is probably associated with the increase in obesity rates, a global health problem linked to an increased risk of developing multiple cancer types. Therefore, the identification of improved, personalized, therapeutic and diagnostic/prognostic tools is urgently needed. MicroRNAs have emerged as promising diagnostic/therapeutic tools for various metabolic/tumoral pathologies but the clinical utility and pathophysiological role of different miRNAs in the interplay between obesity-PCa have been poorly studied. Herein, miR-191–5p levels (plasma and tissue) were analyzed in control and PCa patients by quantitative-PCR and microarrays using one internal (n = 202) and two external (n = 1600) cohorts, and its diagnostic/prognostic/therapeutic potential and functional role was assessed through bioinformatic and clinical association analyses, and in vitro assays (e.g. proliferation/migration/colony-formation, phenotype-rescue). Plasma miR-191–5p was significantly altered in independent PCa cohorts (p < 0.0001), showing a better ability than Prostate Specific Antigen (PSA) to discriminate between patients with and without PCa, especially in the 'grey zone' of PSA (3–10 ng/mL; wherein sensitivity/specificity for diagnosing PCa is significantly compromised). Remarkably, the diagnostic capacity of miR-191–5p drastically increased when obese PCa patients were considered, allowing a strong discriminatory capacity between obese patients with PCa or significant-PCa vs. control-patients and, most importantly, between obese patients with significant-PCa vs. nonsignificant-PCa, thus representing a potential personalized diagnostic/prognostic biomarker in obesity-PCa interplay. Additionally, miR-191–5p act as a tumour-suppressor in PCa cells by regulating critical PCa hallmarks (including proliferation/migration/colony-formation and key oncogenic molecular markers expression) through the posttranscriptional downregulation of TMOD2 (member of the tropomodulin-family). Altogether, our results provide new, compelling evidence supporting the contention that miR-191–5p represents a promising diagnostic/prognostic biomarker and a novel therapeutic tool, especially in the pathological association between PCa and obesity.