Invensys plc (LSE: ISYS) is a British multinational engineering and information technology company headquartered in London, United Kingdom. It was formed in 1999 through the merger of BTR plc and Siebe plc. It has offices in more than 50 countries and its products are sold in around 180 countries.

Invensys is organised into three main divisions. Its Invensys Operations Management division is a global industrial automation supplier and trades under brands including Avantis, Eurotherm, Foxboro, IMServ, InFusion, Triconex, SimSci-Esscor, Skelta, and Wonderware. Its Invensys Controls division is a global supplier of control devices and trades under brands including Drayton, Eberle, Eliwell and Invensys Appliance. Its Invensys Rail division is a major supplier of railway control equipment and software and trades under brands including Dimetronic, Safetran and Westinghouse.

Invensys is listed on the London Stock Exchange and is a constituent of the FTSE 250 Index.

Invensys was formed by the merger of Siebe plc and BTR plc in 1999. From 1999 to 2004, Invensys underwent a major restructuring programme to cut its costs as falling sales and large debts had led to the danger of the company going bankrupt. A major disposal programme, combined with a £2.7bn debt restructuring in 2004 saved the company from collapse. During that time Invensys bought and sold The Baan Corporation.