Prostate cancer (PCa) is the tumour pathology with the second highest incidence among men worldwide. PCa is strongly influenced by obesity (OB), which increases its aggressiveness. Hence, some metabolic drugs like metformin have emerged as potential anti-tumour agents against several endocrine-related cancers. Likewise, a high adherence to the Mediterranean diet has been associated with lower rates of OB and a reduction in PCa aggressiveness since this diet contains phenolic bioactive compounds such as hydroxytyrosol (HT) that is mainly present in extra virgin olive oil. Thus, we decided to analyse the therapeutic potential of the combination of HT + metformin in different PCa cell models. Specifically, combinations of different doses of HT and metformin were evaluated by analysing the proliferation rate of LNCaP, 22Rv1, DU-145, and PC-3 cells using the SynergicFinder method. The results revealed a synergistic effect of HT + metformin in significantly reducing proliferation, especially in LNCaP cells. This anti-tumour effect of HT + metformin was also confirmed in migration and tumoursphere formation assays in LNCaP. The effects on the cell cycle and apoptosis were also assessed by flow-cytometry, and a cycle arrest in the G1 phase and an increase in late apoptosis were observed with the combination of HT + metformin. The phosphorylation levels of critical components of different oncogenic pathways were measured which revealed that the combination of HT + metformin significantly reduced the activity of multiple components of the MAPK, AKT, and TGF-β pathways. Overall, the combination of HT + metformin might represent a new therapeutic avenue for the management of PCa patients, an observation that certainly warrants further investigation through a well-designed clinical trial.