OpenLisp is a programming language in the Lisp family developed by Christian Jullien[1] from Eligis.

OpenLisp is designated an ISLISP implementation, but also contains many Common Lisp-compatible extensions (hashtable, readtable, package, defstruct, sequences, rational numbers) and other libraries (network socket, regular expression, XML, Portable Operating System Interface (POSIX), SQL, Lightweight Directory Access Protocol (LDAP)).

[8] OpenLisp includes an interpreter associated to a read–eval–print loop (REPL), a Lisp Assembly Program (LAP) and a backend compiler for the language C. The main goal of this Lisp version is to implement a fully compliant ISLISP system (when launched with --islisp flag, it is strictly restricted to ISO/IEC 13816:2007(E) specification).

OpenLisp mainly runs in console mode: cmd.exe on Microsoft Windows, and terminal emulator on Unix-based systems.

Small objects of the same type are allocated using a Bibop (BIg Bag Of Pages) memory organization.

OpenLisp claims to be extremely portable, it runs on many operating systems including: Windows, most Unix and POSIX based (Linux, macOS, FreeBSD, OpenBSD, NetBSD, Solaris, HP-UX, AIX, Cygwin, QNX), DOS, OS/2, Pocket PC, OpenVMS, z/OS.

OpenLisp can interact with modules written in C using foreign function interface (FFI), ISLISP streams are extended to support network socket (./net directory includes samples for Hypertext Transfer Protocol (http), JavaScript Object Notation (JSON), Post Office Protocol 3 (POP3), Simple Mail Transfer Protocol (SMTP), Telnet, Rss), a simplified Extensible Markup Language (XML) reader can convert XML to Lisp.

Developer tools include data logging, pretty-printer, profiler, design by contract programming, and unit tests.

[11] The name was chosen in 1993 to replace the MLisp internal code name which was already used by Gosling Emacs (as successor of Mocklisp).