Siemens Plessey was the name given to the Plessey businesses acquired by Siemens AG in 1989.

In 1988 a full merger between Plessey and GEC was rejected. Instead GEC and Plessey merged their communications businesses to form GEC Plessey Telecommunications. In 1989 GEC and Siemens set up a joint holding company, GEC Siemens plc, to acquire Plessey outright. The division of Plessey between these two partners meant the deal met with little regulatory opposition. Siemens acquired Plessey's radar systems, defence systems and traffic control businesses. In context, Plessey was at the time the second largest defence electronics contractor to the Ministry of Defence and the largest supplier of traffic control equipment.

The defence electronics operations became Siemens Plessey Systems (commonly referred to as Siemens Plessey) and Siemens Plessey Electronics Systems (Australia). In 1997 Siemens agreed with British Aerospace and Daimler-Benz Aerospace the takeover of its defence electronics unit, with BAe acquiring the UK operations and DASA the German part of the business.

Siemens Traffic Controls continues to operate as a unit of Siemens.