SYS may indicate:

• SYS (operating system), a Linux distribution
• SYS (command), a DOS command used to make a medium bootable
• SYS, a BASIC command
• SYS Technologies, a information technology company
• ISO 639-3 code of the sinyar language
• IATA code of the Saskylakh Airport
• Stretch-Yawn Syndrome (pandiculation)
• Salford Youth Service
• .sys filename extension

.sys is a filename extension in Microsoft Windows and DOS-type operating system.

The Security Service, also MI5 (Military Intelligence, Section 5), is the United Kingdom's domestic counter-intelligence and security agency and is part of its intelligence machinery alongside the Secret Intelligence Service (MI6), Government Communications Headquarters (GCHQ) and Defence Intelligence (DI). MI5 is directed by the Joint Intelligence Committee (JIC), and the service is bound by the Security Service Act 1989 and the Intelligence Services Act 1994. The service is directed to protect British parliamentary democracy and economic interests, and counter terrorism and espionage within the UK.

sysfs is a virtual file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuring.