Photodynamic therapy (PDT) has attracted extensive attention in recent years as a noninvasive and locally targeted cancer treatment approach. Nanoparticles have been used to improve the solubility and pharmacokinetics of the photosensitizers required for PDT; however, nanoparticles also suffer from many shortcomings including uncontrolled drug release and low tumor accumulation. Herein, we describe a novel biodegradable nanoplatform for the delivery of the clinically used PDT photosensitizer benzoporphyrin derivative monoacid ring A (BPD-MA) to tumors. Specifically, the hydrophobic photosensitizer BPD was covalently conjugated to the amine groups of a dextran-b-oligo (amidoamine) (dOA) dendron copolymer, forming amphiphilic dextran-BPD conjugates that can self-assemble into nanometer-sized micelles in water. To impart additional imaging capabilities to these micelles, superparamagnetic iron oxide nanoparticles (SPIONs) were encapsulated within the hydrophobic core to serve as a magnetic resonance imaging (MRI) contrast agent. The use of a photosensitizer as a hydrophobic building block enabled facile and reproducible synthesis and high drug loading capacity (∼30%, w/w). Furthermore, covalent conjugation of BPD to dextran prevents the premature release of drug during systemic circulation. In vivo studies show that the intravenous administration of dextran-BPD coated SPION nanoparticles results in significant MR contrast enhancement within tumors 24 h postinjection and PDT led to a significant reduction in the tumor growth rate.