Over the past decades, self-assembly has become a pivotal strategy for the construction of tunable supramolecular nanoarchitectures. In particular, supramolecular assemblies from d⁸ transition metal complexes have garnered widespread attention as bioimaging and therapeutic agents owing to their rich physicochemical properties resulting from metallophilic interactions. These supramolecular assemblies exhibit red-shifted absorption and emission, which are critical for imaging and phototherapy because low-energy light can penetrate deeper into tissue and poses less cytotoxic risk compared to ultraviolet light. In cell cultures, these self-assembled nanoaggregates enhance cellular uptake via endocytic pathways. In certain cases, they induce irreversible organelle swelling and membrane permeabilization, leading to organelle dysfunction and cell death, thereby overcoming chemotherapy resistance mechanisms. Furthermore, metallophilic interactions remain stable during in vivo blood circulation, facilitating the formation of self-assembled nanostructures with strong tumor-targeting capabilities. Therefore, supramolecular assemblies offer an innovative and straightforward method for enhancing tumor treatment efficacy. This review highlights recent advances in the development of supramolecular assemblies via metal-metal interactions for use in bioimaging and therapy; summarizes the design strategies, self-assembly processes, photochemical and photophysical properties, bioimaging performance, and therapeutic efficacy of representative supramolecular assemblies; and discusses challenges and opportunities for these assemblies in biological applications.