Pinus radiata bark is a rich source of proanthocyanidins (PAs), a potent and valuable plant antioxidant. This study was performed to evaluate PA extraction conditions with water at various temperatures ranging from 25°C to 120°C. The properties of the water extract (WE) obtained at each temperature were investigated in terms of PA content, molecular weight (MW) distribution, and antioxidant activity. The WE yield was significantly dependent on temperature. The PA yield and the absorbance at 280 nm (an indicator of PA concentration) of WE reached maximum values at 80°C, implying increased extraction of monomeric polyphenols. Gel permeation chromatography results suggested that water extraction above 100°C caused depolymerization of extracted PAs, thereby noticeably reducing MW. It was found that more monomeric polyphenols can be extracted by increasing temperature. The WE antioxidant activity was maximized at 80°C and was dependent to some extent on the degree of polymerization.