The Xerox Alto is a computer system developed at Xerox PARC (Palo Alto Research Center) in the 1970s. It is considered one of the first workstations or personal computers, and its development pioneered many aspects of modern computing. The advances include graphical user interface (GUI), computer mouse, Ethernet networking, and the ability to run multiple applications simultaneously. It is one of the first computers to use a WYSIWYG (What You See Is What You Get) text editor and has a bit-mapped display. The Alto did not succeed commercially, but it had a significant influence on the development of future computer systems.

The Alto was designed for an operating system based on a GUI, later using the desktop metaphor. The first machines were introduced on March 1, 1973, and in limited production starting one decade before Xerox's designs inspired Apple to release the first mass-market GUI computers. The Alto is contained in a relatively small cabinet and uses a custom central processing unit (CPU) built from multiple SSI and MSI integrated circuits. Each machine cost tens of thousands of dollars. Few were built initially, but by the late 1970s, about 1,000 were in use at various Xerox laboratories, and about another 500 in several universities. Total production was about 2,000 systems.

The Alto became well known in Silicon Valley and its GUI was increasingly seen as the future of computing. In 1979, Steve Jobs arranged a visit to Xerox PARC, during which Apple Computer personnel received demonstrations of Xerox technology in exchange for Xerox being able to purchase stock options in Apple. After two visits to see the Alto, Apple engineers used the concepts in developing the Lisa and Macintosh systems.

In 1981, Xerox commercialized a line of office computers, the Star, based on concepts from the Alto. A complete office system including several workstations, storage, and a laser printer cost up to $100,000 (equivalent to $350,000 in 2025). Like the Alto, the Star had little direct impact on the market.

The Alto is the first computer with a graphical operating system, and was built on earlier graphical interface designs. It was conceived in 1972 in a memo written by Butler Lampson, inspired by the oN-Line System (NLS) developed by Douglas Engelbart and Dustin Lindberg at SRI International (SRI). Of further influence was the PLATO education system developed at the Computer-based Education Research Laboratory at the University of Illinois. The Alto was designed mostly by Charles P. Thacker. Industrial Design and manufacturing was sub-contracted to Xerox's Special Programs Group in El Segundo, whose team included program manager Doug Stewart, operations manager Abbey Silverstone, and industrial designer Bob Nishimura. An initial run of 30 units was produced by the Special Programs Group, working with John Ellenby at PARC and Stewart and Silverstone at El Segundo, who were responsible for re-designing the Alto's electronics. Due to the success of the pilot run, the team went on to produce approximately 2,000 units over the next ten years.

Several Xerox Alto chassis are on display at the Computer History Museum in Mountain View, California, one is on display at the Mimms Museum of Technology and Art in Roswell, Georgia, and several are in private hands. Running systems are on display at the System Source Computer Museum in Hunt Valley, Maryland. Charles P. Thacker was awarded the 2009 Turing Award of the Association for Computing Machinery on March 9, 2010, for his pioneering design and realization of the Alto. The 2004 Charles Stark Draper Prize was awarded to Thacker, Alan C. Kay, Butler Lampson, and Robert W. Taylor for their work on Alto.

On October 21, 2014, source code for Alto software, and other resources, were released by the Computer History Museum.

The following description is based mostly on the August 1976 Alto Hardware Manual by Xerox PARC.