Guy Lewis Steele Jr. (/stiːl/; born October 2, 1954) is an American computer scientist who has played an important role in designing and documenting several computer programming languages and technical standards.

Steele was born in Missouri and graduated from the Boston Latin School in 1972. He received a Bachelor of Arts (BA) in applied mathematics from Harvard University (1975) and a Master's degree (MS) and Doctor of Philosophy (PhD) from Massachusetts Institute of Technology (MIT) in computer science (1977, 1980). He then worked as an assistant professor of computer science at Carnegie Mellon University and a compiler implementer at Tartan Laboratories. Then he joined the supercomputer company Thinking Machines, where he helped define and promote a parallel computing version of the Lisp programming language named *Lisp (Star Lisp) and a parallel version of the language C named C*.

In 1994, Steele joined Sun Microsystems and was invited by Bill Joy to become a member of the Java team after the language had been designed, since he had a track record of writing good specifications for extant languages.^{[citation needed]} He was named a Sun Fellow in 2003. Steele joined Oracle in 2010 when Oracle acquired Sun Microsystems.

While at MIT, Steele published more than two dozen papers with Gerald Jay Sussman on the subject of the language Lisp and its implementation (the Lambda Papers). One of their most notable contributions was the design of the language Scheme.

Steele also designed the original command set of Emacs and was the first to port TeX (from WAITS to ITS). He has published papers on other subjects, including compilers, parallel processing, and constraint languages. One song he composed has been published in the official journal of the Association for Computing Machinery Communications of the ACM (CACM) ("The Telnet Song", April 1984, a parody of the behavior of a series of PDP-10 TELNET implementations written by Mark Crispin).

Steele has served on accredited technical standards committees, including: Ecma International (formerly European Computer Manufacturers Association (ECMA)) TC39 (for the language ECMAScript, for which he was editor of the first edition), X3J11 (for C), and X3J3 (for Fortran) and is, as of 2019^[update], chairman of X3J13 (for Common Lisp). He was also a member of the Institute of Electrical and Electronics Engineers (IEEE) working group that produced the IEEE Standard for the language Scheme, IEEE Std 1178–1990. He represented Sun Microsystems in the High Performance Fortran Forum, which produced the High Performance Fortran specification in May, 1993.

In addition to specifications of the language Java, Steele's work at Sun Microsystems has included research in parallel algorithms, implementation strategies, and architecture and software support. In 2005, Steele began leading a team of researchers at Sun developing a new language named Fortress, a high-performance language designed to obsolete Fortran.

Steele participated in the development of the Verse programming language designed by Epic Games.