MSI Plessey (also known as Modified Plessey) is a numeric-only, variable-length symbology that is a variant of Plessey Code, which was originally developed by the Plessey Company of England in 1971.

MSI Data Corporation developed MSI Plessey to address the disadvantages of Plessey Code: it does not encode numbers efficiently, and it is one of the most error-prone of all barcode symbologies. The second check digit that MSI Data Corporation added as an option improved the code's reliability enough that MSI Plessey has continued to be used in specialty applications.

MSI Plessey has typically been used for inventory tracking, such as on the front edge of shelves in supermarkets and other warehouse environments to indicate where products are stored. It has also been used to identify storage containers.

MSI Plessey is currently maintained by Symbol Technologies, Inc.

The symbol comprises the following elements:

• Leading quiet zone
• Start character (wide bar, narrow space)
• Encoded data
• One or two check digits
• Stop character (narrow bar, wide space, narrow bar)
• Trailing quiet zone

This symbology supports all numeric digits (0-9).

MSI Plessey contains an optional check digit or pair of check digits that are based on one of the following algorithms:

• Modulo 10
• Modulo 11
• Modulo 1010 (two modulo 10 calculations)
• Modulo 1110 (one modulo 11 and one modulo 10 calculation)