The objective of this study was to develop an active-targeted, pH-responsive albumin-photosensitizer conjugate as a theranostic agent. Herein, a porphyrin derivative photosensitizer, pheophorbide-a (PheoA), was conjugated to bovine serum albumin (BSA) via a cis-aconityl linkage, and the conjugate was then linked with polyethylene glycosylated folate to improve targeting ability. Further, BSA-c-PheoA and folate (FA)-BSA-c-PheoA at a ratio of 2 : 1 were self-assembled to form nanoparticles with a mean hydrodynamic diameter of 121.47 ± 11.60 nm. The release study exhibited that the photosensitizer was released 4.5-fold faster at pH 5.0 than at pH 7.4 when incubated for 24 h. Cellular uptake results showed that the FA-BSA-c-PheoA nanoparticles were readily taken up by B16F10 and MCF7 cancer cells. In vitro phototoxicity results showed that FA-BSA-c-PheoA NPs have higher efficacy on cancer cells compared to simple BSA-c-PheoA NPs. In vivo bioimaging results exhibited that FA-BSA-c-PheoA NPs greatly accumulated into the tumor area as compared to free PheoA. These results show that our prepared FA-BSA-c-PheoA NPs have the potential to be applied as theranostic agents in photodynamic therapy and photodiagnosis of cancer.