Acorn flours from the six Mediterranean Quercus species (Quercus cerris, Quercus petraea, Quercus robur, Quercus ilex, Quercus pubescens, and Quercus rotundifolia) were systematically fractionated for polyphenols using ultrasonic-assisted extraction with seven solvent systems varying in polarity and composition (water at 20 and 40 min; acetone, ethanol, and methanol at 20% and/or 70% v/v). The total polyphenol content (TPC), non-tannic phenolic content (NTPC), tannin content (TC), antioxidant potential (DPPH, ABTS, and FRAP), and individual phenolic profiles through high-performance liquid chromatography (HPLC) were determined. The results showed that the botanical species primarily determined the TPC and TC, while the solvent composition significantly influenced the NTPC yield. Q. cerris yielded the highest average TPC (105.1 ± 3.8 mg GAE/g) and TC, supported by a gallotannin-dominated profile. Conversely, Q. rotundifolia exhibited the lowest values but the highest NTPC/TPC ratio (32.0%). Q. ilex featured species-exclusive ellagitannins, while Q. pubescens showed the highest specific antioxidant activity. For the targeted recovery, 20% acetone is recommended for tannins and 70% ethanol for the non-tannic fractions. These findings establish a species-resolved framework for valorizing acorn flours as functional ingredients, identifying high-tannin species requiring detannification and “sweet” varieties suitable for direct food application.