Photodynamic therapy, a procedure that uses a photosensitizer to enable light therapy selectively at diseased sites, remains underutilized in oncological clinic. To further improve its cancer selectivity, we developed a polymeric nanosystem by conjugating a photosensitizer IRDye 700DX (IR700) and cancer targeting RGD peptide to 8-arm polyethylene glycol (PEG). The resulting nanoconjugates (RGD-8PEG-IR700) exhibited a hydrodynamic size of 6.6 nm with narrow distribution of size. The targeted nanoconjugates showed significantly higher intracellular uptake of IR700 in integrin αvβ3-expressing A375 and SKOV3 cells when compared with nontargeted control 8PEG-IR700, and an excess amount of RGD peptides could abolish this enhancement, indicating a receptor-mediated uptake mechanism for the targeted polymer conjugates. Phototoxicity studies indicated that RGD-8PEG-IR700 produced massive cell killing in A375 cells after photoirradiation with an IC₅₀ value of 57.8 nM for IR700. In contrast, free IR700 and the control 8PEG-IR700 conjugates did not produce any phototoxicity at the concentrations up to 1 μM IR700. Upon photoirradiation, the RGD-8PEG-IR700 could produce sufficient singlet oxygen in the cells and induced cell apoptosis. The studies with three-dimensional tumor spheroids showed that they penetrated tumor spheroids deeply and produced strong phototoxicity. Thus, we conclude that the polymer nanoconjugates may provide a promising delivery system for targeted photodynamic therapy of cancers due to their small size, cancer cell specificity, and minimal side effects.