PCI-X, short for Peripheral Component Interconnect eXtended, is a computer bus and expansion card standard that enhances the 32-bit PCI local bus for higher bandwidth demanded mostly by servers and workstations. It uses a modified protocol to support higher clock speeds (up to 133 MHz), but is otherwise similar in electrical implementation. PCI-X 2.0 added speeds up to 533 MHz,^: 23 with a reduction in electrical signal levels.

The slot is physically a 3.3 V PCI slot, with exactly the same size, location and pin assignments. The electrical specifications are compatible, but stricter. However, while most conventional PCI slots are the 85 mm long 32-bit version, most PCI-X devices use the 130 mm long 64-bit slot, to the point that 64-bit PCI connectors and PCI-X support are seen as synonymous.

PCI-X is in fact fully specified for both 32- and 64-bit PCI connectors,^: 14 and PCI-X 2.0 added a 16-bit variant for embedded applications.^: 22

It has been replaced in modern designs by the similar-sounding PCI Express (officially abbreviated as PCIe), with a completely different physical connector and a very different electrical design, having one or more narrow but fast serial connection lanes instead of a number of slower connections in parallel.