Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set computers (CISC) and to be a highly competitive RISC processor for Unix workstations and similar markets.

Alpha was implemented in a series of microprocessors originally developed and fabricated by DEC. These microprocessors were most prominently used in a variety of DEC workstations and servers, which eventually formed the basis for almost all of their mid-to-upper-scale lineup. Several third-party vendors also produced Alpha systems, including PC form factor motherboards.

Operating systems that support Alpha included OpenVMS (formerly named OpenVMS AXP), Tru64 UNIX (formerly named DEC OSF/1 AXP and Digital UNIX), Windows NT (discontinued after NT 4.0; and prerelease Windows 2000 RC2), Linux (Debian, SUSE, Gentoo and Red Hat), BSD UNIX (NetBSD, OpenBSD and FreeBSD up to 6.x), Plan 9 from Bell Labs, and the L4Ka::Pistachio kernel. A port of Ultrix to Alpha was carried out during the initial development of the Alpha architecture, but was never released as a product.

The Alpha architecture was sold, along with most parts of DEC, to Compaq in 1998. Compaq, already an Intel x86 customer, announced that they would phase out Alpha in favor of the forthcoming Hewlett-Packard/Intel Itanium architecture, and sold all Alpha intellectual property to Intel, in 2001, effectively killing the product. Hewlett-Packard purchased Compaq in 2002, continuing development of the existing product line until 2004, and selling Alpha-based systems, largely to the existing customer base, until April 2007.

Digital Equipment Corporation (DEC) during the 1980s was very successful with its 32-bit CISC-based VAX computers running the VMS operating system. By the mid-1980s the company nonetheless agreed with the industry consensus that new RISC technology was much more efficient than CISC. Many competing internal projects began in response, causing delays and uncertainty.

Alpha emerged from an earlier RISC project named Parallel Reduced Instruction Set Machine (PRISM), itself the product of several earlier projects. PRISM was intended to be a flexible design, supporting Unix-like applications, and Digital's existing VAX/VMS software, after minor conversion. A new operating system named MICA would support both ULTRIX and VAX/VMS interfaces on a common kernel, allowing software for both platforms to be easily ported to the PRISM architecture.

Started in 1985, the PRISM design was continually changed during its development in response to changes in the computer market, leading to lengthy delays in its introduction. It was not until the summer of 1987 that it was decided that it would be a 64-bit design, among the earliest such designs in a microprocessor format. In October 1987, Sun Microsystems introduced the Sun-4, their first workstation using their new SPARC processor. The Sun-4 runs about three to four times as fast as their latest Sun-3 designs using the Motorola 68020, and any Unix offering from DEC. The plans changed again; PRISM was realigned once again as a 32-bit part and aimed directly at the Unix market. This further delayed the design.

Having watched the PRISM delivery date continue to slip, and facing the possibility of more delays, a team in the Palo Alto office decided to design their own workstation using another RISC processor. It selected the MIPS R2000 and built the first DECstation running Ultrix in a period of 90 days. This sparked off an acrimonious debate within the company, which came to a head in a July 1988 management meeting. PRISM appeared to be faster than the R2000, but the R2000 machines could be in the market by January 1989, a year earlier than PRISM. When this proposal was accepted, one of the two original roles for PRISM disappeared. The decision to make a VMS PRISM had already ended by this point, so there was no remaining role. PRISM was cancelled at the meeting.