A complex is a whole that comprehends a number of parts, especially one with interconnected or mutually related parts. It may refer to:

• In chemistry, a complex, also called a coordination compound or metal complex, is a structure consisting of a central atom or molecule weakly connected to surrounding atoms or molecules.

• In psychology, a complex is a group of mental factors that are unconsciously associated by the individual with a particular subject and influence the individual's attitude and behavior.