Precision Architecture RISC (PA-RISC) or Hewlett Packard Precision Architecture (HP/PA or simply HPPA), is a general purpose computer instruction set architecture (ISA) developed by Hewlett-Packard from the 1980s until the 2000s.

The architecture was introduced on 26 February 1986, when the HP 3000 Series 930 and HP 9000 Model 840 computers were launched featuring the first implementation, the TS1. HP stopped selling PA-RISC-based HP 9000 systems at the end of 2008 but supported servers running PA-RISC chips until 2013. PA-RISC was succeeded by the Itanium (originally IA-64) ISA, jointly developed by HP and Intel.

In the late 1980s, HP was building four series of computers, all based on CISC CPUs. One line was the IBM PC compatible Intel i286-based Vectra Series, started in 1986. All others were non-Intel systems. One of them was the HP Series 300 of Motorola 68000-based workstations, another Series 200 line of technical workstations based on a custom silicon on sapphire (SOS) chip design, the SOS based 16-bit HP 3000 classic series, and finally the HP 9000 Series 500 minicomputers, based on their own (16- and 32-bit) FOCUS microprocessor.

The Precision Architecture is the result of what was known inside Hewlett-Packard as the Spectrum program. HP planned to use Spectrum to move all of their non-PC compatible machines to a single RISC CPU family.

In early 1982, work on the Precision Architecture began at HP Laboratories, defining the instruction set and virtual memory system. Development of the first TTL implementation started in April 1983. With simulation of the processor having completed in 1983, a final processor design was delivered to software developers in July 1984. Systems prototyping followed, with "lab prototypes" being produced in 1985 and product prototypes in 1986.

The first processors were introduced in products during 1986, with the first HP 9000 Series 840 units shipping in November of that year. They were also used in a new series of HP 3000 machines in the late 1980s: the 930 and 950, commonly known at the time as Spectrum systems, the name given to them in the development labs. These machines ran MPE-XL, whereas HP 9000 machines employing the PA-RISC processor ran the HP-UX version of Unix. The first implementation of the Precision Architecture was the TS1, a central processing unit built from discrete transistor–transistor logic (74F TTL) devices. Later implementations were multi-chip VLSI designs fabricated in NMOS processes (NS1 and NS2) and CMOS (CS1 and PCX), beginning with the HP 3000 Series 950, HP 9000 Model 850S and HP 9000 Model 825, introduced in late 1987.

The HP Precision Architecture has thirty-two 32-bit integer registers, sixteen 64-bit floating-point registers, and has a single branch delay slot. This means that the instruction immediately following a branch instruction is executed before the program's control flow is transferred to the target instruction of the branch. An HP Precision processor also includes a Processor Status Word (PSW) register. The PSW register contains various flags that enable virtual addressing, protection, interruptions, and other status information. The number of floating-point registers was doubled in the 1.1 version to 32 once it became apparent that 16 were inadequate and restricted performance. The architects included Allen Baum, Hans Jeans, Michael J. Mahon, Ruby Bei-Loh Lee, Russel Kao, Steve Muchnick, Terrence C. Miller, David Fotland, and William S. Worley.

Other operating systems ported to the PA-RISC architecture include Linux, OpenBSD, NetBSD, OSF/1, NeXTSTEP, and ChorusOS.