Open Inventor, originally IRIS Inventor, is a C++ object-oriented retained mode 3D graphics toolkit designed by SGI to provide a higher layer of programming for OpenGL. Its main goals are better programmer convenience and efficiency. Open Inventor exists as both proprietary software and free and open-source software, subject to the requirements of the GNU Lesser General Public License (LGPL), version 2.1.

In the late 1980s, Wei Yen and Rikk Carey initiated the IRIS Inventor project to create a toolkit for easier 3D graphics application development. The project led to the creation of Open Inventor (OI), which provided a common base layer for 3D programming. OI simplified the creation of 3D applications but faced issues like slower performance and limited file format compatibility. A different approach, OpenGL Performer, prioritized performance optimization but remained proprietary.

In the mid-1990s, SGI attempted to merge the two systems into Cosmo 3D and later OpenGL++, but both projects were ultimately discontinued. Open Inventor was licensed to third-party developers, and in August 2000, it was released under the LGPL open source license by SGI. The Open Inventor API has been widely used in scientific and engineering visualization systems, with Thermo Scientific Open Inventor still under active development for various applications.