| X++ | — | — | Microsoft |
| X-BASIC | — | — | Hudson Soft, Sharp Corporation |
| X10 | concurrent computing, array programming, object-oriented programming | 2004 | IBM |
| X86 assembly language | — | — | — |
| XACML | declarative programming, semi-structured data | — | OASIS |
| XBase++ | object-oriented programming | 1997 | — |
| XBasic | — | — | — |
| XBLite | procedural programming | — | — |
| XC | concurrent computing | 2005 | XMOS |
| Xerox Escape Sequence | — | 1982 | — |
| XGQL | — | — | — |
| XHarbour | procedural programming | 2001 | — |
| XL | multi-paradigm programming, imperative programming, functional programming | 2000 | Christophe de Dinechin |
| XL | — | 2001 | — |
| XLNT | — | 1997 | Advanced Systems Concepts, Inc. |
| XML | — | 1998 | World Wide Web Consortium |
| XML User Interface Language | — | 1999 | Mozilla Foundation |
| XOD | declarative programming, functional reactive programming, visual programming language, dataflow programming | 2016 | — |
| Xojo | object-oriented programming | 1996 | — |
| XOTcl | event-driven programming, functional programming, imperative programming, multi-paradigm programming, object-oriented programming | 2000 | — |
| XPath | — | 1999 | World Wide Web Consortium |
| XPL | structured programming, procedural programming | 1967 | — |
| XPL0 | procedural programming | 1976 | — |
| XProc | declarative programming | — | — |
| XProfan | — | 1990 | — |
| XProfan | — | 1990 | — |
| XQuery | modular programming, declarative programming, functional programming | 2007 | — |
| XSB | logic programming | — | — |
| XSLT | declarative programming | 1999 | World Wide Web Consortium |
| Xtend | functional programming, imperative programming, object-oriented programming | 2011 | TypeFox GmbH |
| ΛProlog | logic programming | — | — |