Irreducible complexity (IC) is the argument that certain biological systems with multiple interacting parts would not function if one of the parts were removed, so supposedly could not have evolved by successive small modifications from earlier less complex systems through natural selection, which would need all intermediate precursor systems to have been fully functional. Irreducible complexity has become central to the creationist concept of intelligent design (ID), but the concept of irreducible complexity has been rejected by the scientific community, which regards intelligent design as pseudoscience. Irreducible complexity and Specified complexity are the two main arguments used by intelligent-design proponents to support their version of the theological argument from design.

The IC argument was already featured in creation science by the mid 1960s. In 1993 Michael Behe, a professor of biochemistry at Lehigh University, presented a variation of the same argument, supporting ID, in a revised version of the school textbook Of Pandas and People. The expression irreducible complexity appeared in his 1996 book Darwin's Black Box and he said it made evolution through natural selection of random mutations impossible, or extremely improbable. This was based on the mistaken assumption that evolution relies on improvement of existing functions, ignoring how complex adaptations originate from changes in function, and disregarding published research. Evolutionary biologists have published rebuttals showing how systems discussed by Behe can evolve, and examples documented through comparative genomics show that complex molecular systems are formed by the addition of components as revealed by different temporal origins of their proteins.

In the 2005 Kitzmiller v. Dover Area School District trial, Behe gave testimony on the subject of irreducible complexity. The court found that "Professor Behe's claim for irreducible complexity has been refuted in peer-reviewed research papers and has been rejected by the scientific community at large."