In the field of telecommunications, a Clos network is a kind of multistage circuit-switching network which represents a theoretical idealization of practical, multistage switching systems. It was invented by Edson Erwin in 1938 and first formalized by the American engineer Charles Clos in 1952.

By adding stages, a Clos network reduces the number of crosspoints required to compose a large crossbar switch. A Clos network topology (diagrammed below) is parameterized by three integers n, m, and r: n represents the number of sources which feed into each of r ingress stage crossbar switches; each ingress stage crossbar switch has m outlets; and there are m middle stage crossbar switches.

Circuit switching arranges a dedicated communications path for a connection between endpoints for the duration of the connection. This sacrifices total bandwidth available if the dedicated connections are poorly utilized, but makes the connection and bandwidth more predictable, and only introduces control overhead when the connections are initiated, rather than with every packet handled, as in modern packet-switched networks.

When the Clos network was first devised, the number of crosspoints was a good approximation of the total cost of the switching system. While this was important for electromechanical crossbars, it became less relevant with the advent of VLSI, wherein the interconnects could be implemented either directly in silicon, or within a relatively small cluster of boards. Upon the advent of complex data centers, with huge interconnect structures, each based on optical fiber links, Clos networks regained importance. A subtype of Clos network, the Beneš network, has also found recent application in machine learning.