Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each other
* Complex (psychology), a core pattern of emotions etc. in the personal unconscious organized around a common theme such as power or status
Complex may also refer to:
Arts, entertainment and media
* Complex (English band), formed in 1968, and their 1971 album ''Complex''
* Complex (band), a Japanese rock band
* ''Complex'' (album), by Montaigne, 2019, and its title track
* ''Complex'' (EP), by Rifle Sport, 1985
* "Complex" (song), by Gary Numan, 1979
* "Complex", a song by Katie Gregson-MacLeod, 2022
* "Complex" a song by Be'O and Zico, 2022
* Complex Networks, publisher of the now-only-online magazine ''Complex''
Biology
* Protein–ligand complex, a complex of a protein bound with a ligand
* Exosome complex, a multi-p ...