Kay has been a leading figure in the development of modern programming languages, including Smalltalk, which he helped invent in the 1970s.
An industry force for more than 30 years, Kay was one of the founding researchers at Xerox's Palo Alto Research Centre, where he helped create early graphical user interface (GUI) technologies that still influence software design today.
At HP, Kay will work on development of software platforms and distributed applications, including those based on open-source code, the company said.
Kay will also continue his work with Viewpoints Research Institute, a non-profit organisation he co-founded in 2001 that aims to change how children are taught maths and science.
One of the group's tools is Squeak, a programming language Kay helped develop that relies on direct image manipulation and scripting. Described as an "idea processor", Squeak is intended to let users, including children, translate concepts to programs.
Kay's work on the evolution of software and programming could pay off for HP, according to one analyst.
"One of the great challenges we face today is that we have all this great computing power and all these ideas, but we're unable to convert those things to software unless we're programmers. How do you make it easier for people to write software? I think that's a very big area of opportunity," said Gartner analyst Martin Reynolds.