This study was performed to evaluate stabilization of extracellular matrix (ECM) proteins by polyphenolic fractions in hot water extract (HWE) from Pinus radiata bark against enzymatic degradation and radicals. To obtain polyphenolic fractions with different molecular weight (MW) distribution, HWE was fractionated into monomeric polyphenols (MPP), oligomeric proanthocyanidin (OPA), and polymeric proanthocyanidin (PPA) fractions by monitoring the UV–visible spectrum of the eluted fractions. For HWE, MPP, OPA, and PPA, the collagen (15 mg)-stabilizing capacities at 192 μg/mL were 86, 64, 76, and 88%, respectively, while the elastin-stabilizing capacities at 45.5 μg/mL were 37.6, 1.5, 10.8, and 41.8%, respectively. Among these fractions with different MW distribution, PPA played a significant role in the ECM (collagen and elastin) stabilization in a dose-dependent manner. It could be considered that adsorption of PPA onto the ECM is a significant mechanism of polyphenols for collagen protection. 1,1-Diphenyl-2-picrylhydrazyl (DPPH) free radical- and 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) (ABTS•+)-scavenging activity of PPA adsorbed onto collagen showed a dose-dependent increase.