Linux on IBM Z, Linux on zSystems, or zLinux is the collective term for the Linux operating system compiled to run on IBM mainframes, especially IBM Z, zSystems, and LinuxONE servers. Similar terms which imply the same meaning are Linux/390, Linux/390x, etc. The three Linux distributions certified for usage on the IBM Z hardware platform are Red Hat Enterprise Linux, SUSE Linux Enterprise Server, and Ubuntu.

Linux on IBM Z originated as two separate efforts to port Linux to IBM's System/390 servers. The first effort, the "Bigfoot" project, developed by Linas Vepstas in late 1998 through early 1999, was an independent distribution and has since been abandoned. IBM published a collection of patches and additions to the Linux 2.2.13 kernel on December 18, 1999, to start today's mainline Linux on IBM Z. Formal product announcements quickly followed in 2000, including the Integrated Facility for Linux (IFL) engines. Think Blue Linux was an early mainframe distribution consisting mainly of Red Hat packages added to the IBM kernel. Commercial Linux distributors introduced mainframe editions very quickly after the initial kernel work. The first lines of mainframes supported by Linux enterprise distributions were System/390 G5, G6, and Multiprise 3000.

IBM manager Karl-Heinz Strassemeyer of Böblingen in Germany was the main lead to get Linux running on S/390.

At the start of IBM's involvement, Linux patches for S/390 included some object code only (OCO) modules, without source code. Soon after, IBM replaced the OCO modules with open source modules. Linux on IBM Z is free software under the GNU General Public License.

According to IBM, as of May 2006^[update], over 1,700 customers were running Linux on their mainframes; some examples are Nomura Securities, Home Depot, and the University of Pittsburgh.

Virtualization is required by default on IBM Z; there is no option to run Linux on IBM Z without some degree of virtualization. (Only the very first 64-bit mainframe models, the z900 and z800, included a non-virtualized "basic mode".) The first layer virtualization is provided by the Processor Resource and System Manager (PR/SM) to deploy one or more Logical Partitions (LPARs). Each LPAR supports a variety of operating systems, including Linux on IBM Z. A hypervisor called z/VM can also be run as the second layer virtualization in LPARs. This allows an LPAR to run as many virtual machines (VMs) as can be supported by the resources assigned to the LPAR. KVM on IBM Z is another hypervisor option.

When Linux applications in an LPAR access data and applications in other LPARs, such as CICS, IBM Db2, IMS, Linux, and other mainframe subsystems running on the same physical mainframe, they can utilize HiperSockets, which are memory-only TCP/IP connections. As compared to TCP/IP over standard network interface controllers (NICs, also known as Open System Adapters (OSAs) in mainframes), HiperSockets can improve end-user responsiveness (reduce network latency and processing overhead), security (since there is no network connection to intercept), and reliability (since there is no network connection to lose).

With the zEC12, zBC12, and later models, the HiperSocket concept is extended beyond the physical machine boundary via an RDMA over Converged Ethernet (RoCE) adapter to facilitate a secure and high-speed inter-system communication. Applications in LPAR A in system A can thus use HiperSockets to communicate with applications in LPAR B in system B to ensure the security and performance attributes.^{[citation needed]}