Background: High residual platelet reactivity (HRPR) during dual antiplatelet therapy (DAPT) may impact clinical outcomes following percutaneous coronary interventions (PCI). However, whether any biomarkers assessed before PCI at DAPT loading may predict delayed maintenance HRPR is not clear. Objective: The aim of this study was to determine whether conventional clinical or laboratory indices at loading before stenting may predict HRPR at 6 months of maintenance DAPT. Methods: The study was designed on a single-center prospective cohort, and included 94 pre-PCI patients. All patients underwent elective PCI with drug-eluting stent implantation, and received DAPT with aspirin and clopidogrel. Platelet reactivity was assessed with 5 μmol/L of adenosine diphosphate-induced light transmission aggregometry before PCI, but after 24 h of DAPT loading, and repeated at 6 months. Baseline clinical characteristics, CYP2C19 polymorphism, C-reactive protein, soluble P-selectin, CD40L, interleukin-6, PAI-1 levels, and von Willebrand factor activity were analyzed. Results: The incidence (light transmission aggregometry <50%) of prestent HRPR was 16%. By univariate regression, body mass index (BMI; p = 0.02), total cholesterol (p = 0.01), low-density lipoproteins (p = 0.004), CYP2C19*2 allele carriage (p = 0.006), soluble P-selectin (p = 0.009), and von Willebrand factor (p = 0.04) were linked to future HRPR. However, multivariate regression analysis suggested that only BMI and P-selectin were independent predictors of HRPR. Conclusions: Platelet reactivity before elective stenting is associated with numerous biomarkers; however, only BMI and soluble P-selectin were independent predictors of future HRPR during maintenance-phase DAPT. This may be important for future tailored antiplatelet strategies in patients with metabolic syndrome and diabetics.