The effective administration of therapeutic proteins has received increased attention for the treatment of various diseases. Encapsulation of these proteins in various matrices, as a method of protein structure and function preservation, is a widely used approach that results in maintenance of the protein's function. However, targeted delivery and tracking of encapsulated therapeutic proteins to the affected cells is still a challenge. In an effort to advance the targeted delivery of a functional apoptosis-initiating protein (cytochrome c) to cancer cells, we formulated theranostic polymeric nanoparticles for the simultaneous encapsulation of cytochrome c and a near-infrared dye to folate-expressing cancer cells. The polymeric nanoparticles were prepared using a novel water-soluble hyperbranched polyhydroxyl polymer that allows for dual encapsulation of a hydrophilic protein and an amphiphilic fluorescent dye. Our protein therapeutic cargo is the endogenous protein cytochrome c, which upon cytoplasmic release, initiates an apoptotic response leading to programmed cell death. Results indicate that encapsulation of cytochrome c within the nanoparticle's cavities preserved the protein's enzymatic activity. The potential therapeutic property of these nanoparticles was demonstrated by the induction of apoptosis upon intracellular delivery. Furthermore, targeted delivery of cytochrome c to folate-receptor-positive cancer cells was achieved via conjugation of folic acid to the nanoparticle's surface, whereas the nanoparticle's theranostic properties were conferred via the coencapsulation of cytochrome c and a fluorescent dye. Considering that these theranostic nanoparticles can carry an endogenous cellular apoptotic initiator (cytochrome c) and a fluorescent tag (ICG) commonly used in the clinic, their use and potential translation into the clinic is anticipated, facilitating the monitoring of tumor regression.