Abstract: In the present investigation, the optimization of a pressurized solid–liquid extraction of polyphenols from Pinus pinaster Aiton needles and bark residues has been carried out using new deep eutectic solvent systems (DES). A Box–Behnken design with response surface methodology was implemented, with six variables, including solid:liquid ratio, water content, temperature, extraction time, reactor closure state, and catalyst concentration. The DES consisting of levulinic acid: formic acid exhibited the highest content, and the optimal conditions were determined to be 0.10 g of residue in 10 mL of solvent (without water) and without catalyst, at 60 °C for 5 min. The method demonstrated excellent repeatability and intermediate precision, with a coefficient of variation of less than 5%. In addition, the antimicrobial and antioxidant activities of the polyphenol-enriched extracts were evaluated, obtaining a total reduction in S. aureus cell growth and remarkable antioxidant levels of 144.72 ± 4.24 and 161.01 ± 4.70 mg Trolox equivalents/gram for needle and bark residues, respectively. Under the optimized conditions, the new DES are remarkably superior to conventional solvents and can be recycled and reused, demonstrating that these systems have a promising environmental profile, making them valuable alternatives for biomass fractionation.