A ternary complex is a transient functional assembly formed by the cooperative binding of three biological molecules (e.g., two proteins and a small molecule, or a protein–nucleic acid–ligand complex). Its core characteristic is binding cooperativity: the addition of the third component can significantly enhance or alter the interaction efficiency and function of the first two. In basic research, it is critical for elucidating the mechanisms of transcriptional regulation (e.g., DNA–transcription factor–coregulator complexes) and signal transduction (e.g., scaffold protein-mediated complexes). In the field of novel drug discovery, it directly determines the efficacy of targeted protein degraders such as PROTACs. By inducing the formation of the “target protein–PROTAC–E3 ligase” ternary complex to trigger specific protein degradation, it represents a core target for evaluating drug potency and optimizing molecular design.