IBM System z9 is a line of IBM mainframe computers. The first models were available on September 16, 2005. The System z9 also marks the end of the previously used eServer zSeries naming convention. It was also the last mainframe computer that NASA ever used.

System z9 is a mainframe using the z/Architecture, previously known as ESAME. z/Architecture is a 64-bit architecture which replaces the previous 31-bit-addressing/32-bit-data ESA/390 architecture while remaining completely compatible with it as well as the older 24-bit-addressing/32-bit-data System/360 architecture. The primary advantage of this arrangement is that memory intensive applications like DB2 are no longer bounded by 31-bit memory restrictions while older applications can run without modifications.

With the announcement of the System z9 Business Class server, IBM has renamed the System z9 109 as the System z9 Enterprise Class server. IBM documentation abbreviates them as the z9 BC and z9 EC, respectively.

There are several functional enhancements in the System z9 compared to its zSeries predecessors. Some of the differences include:

The Support Element is the most direct and lowest level way to access a mainframe. It circumvents even the Hardware Management Console and the operating system running on the mainframe. The HMC is a PC connected to the mainframe and emulates the Support Element. All preceding zSeries mainframes used a modified version of OS/2 with custom software to provide the interface. System z9's HMC no longer uses OS/2, but instead uses a modified version of Linux with an OS/2 lookalike interface to ease transition as well as a new interface. Unlike the previous HMC application on OS/2, the new HMC is web-based which means that even local access is done via a web browser. Remote HMC access is available, although only over an SSL encrypted HTTP connection. The web-based nature means that there is no longer a difference between local console access and remote access, which means a remote user potentially has full control if authorized, allowing more flexibility for locating systems within data centers. IBM refers to the new HMC as a "closed platform" which does not allow the user to install software or access the command line interface to increase security and stability. The HMC is also firewalled by default with a minimal number of open ports for remote access.

Program Directed Re-IPL is a new feature for Linux on System z9. It allows Linux systems running in an LPAR to re-IPL (reboot) themselves without operator intervention. This is accomplished by the System z9 storing the device and load parameters used to initially IPL the system.

DB2, VSAM, and other data storage formats achieve greater I/O performance thanks to a new System z9 feature called a MIDAW. Also, the System z9 introduces the zIIP, a new type of processor that accelerates certain specific DB2 tasks. Modified Indirect Data Address Words (MIDAWs) are a channel programming capability of the IBM System z9 processor range, and all subsequent ranges. The MIDAW facility is an extension to the pre-existing Indirect Data Address Word (IDAW) channel programming capability, providing support for more efficient FICON channel programs. MIDAWs allow ECKD channel programs to read and write to many storage locations using one channel command, which means fewer signals up and down the channel are required to transfer the same amount of data. This reduction is particularly noticeable for Extended Format data sets, accessed through Media Manager. Examples include Extended Format Sequential data sets, Extended Format VSAM data sets and certain types of DB2 tablespaces. While each of these data set organizations have alternatives, each has a distinct set of advantages, whether in the area of performance, space saving (through hardware-assisted data compression), or scalability (by allowing an individual data set to exceed 4 GiB).

Java 1.4 and higher support both 32-bit and 64-bit operation on z9. The System z9 also supports the zAAP processor, which allows most of the Java workload to be offloaded from the normal instruction processors. Java workloads executed by the zAAP processor do not count towards the IBM-rated capacity of the z9. This reduces the z9's total cost of ownership compared with other IBM platforms, as otherwise IBM would raise a customer's (software) license fees after installing an additional (hardware) processor. The zAAP also enables integration of new Java based Web applications with core z/OS backend database environment for high performance, reliability, availability, and security.