Polymer nanoparticles have gained significant attention as potential drug carriers for anticancer agents and molecular imaging. Biocompatible gold nanoparticles (AuNPs) were synthesized using chitosan oligosaccharide (COS) as a reducing and stabilizing agent and were subsequently loaded with paclitaxel (PTX) to demonstrate their use in drug delivery and photoacoustic imaging (PAI) of MDA-MB-231 cells. Paclitaxel-loaded chitosan oligosaccharide-stabilized gold nanoparticles (PTX-COS AuNPs) were spherical in shape with an average particle size of 61.86 ± 3.01 nm. PTX-COS AuNPs showed sustained and pH-dependent drug release profiles and exhibited strong cytotoxic effect against MDA-MB-231 cells through the induction of apoptosis with improved reactive oxygen species (ROS) generation and altered mitochondrial membrane potential (MMP) level. The cellular internalization of PTX-COS AuNPs was proven by fluorescence microscopy as well as flow cytometry. PTX-COS AuNPs were also evaluated as a new class of optical contrast agents for photoacoustic imaging (PAI). To the best of our knowledge, this is the first report that describes the use of PTX-COS AuNPs as novel agents for drug delivery and PAI of cancer cells. These results exposed the promising potential of PTX-COS AuNPs in the field of drug delivery, molecular imaging, and cancer therapy in the near future.