Project Vela was a United States Department of Defense project to monitor Soviet Union compliance with the 1963 Partial Test Ban Treaty. The treaty banned the testing of nuclear weapons in the atmosphere, in outer space, and underwater, but permitted underground testing.

In August 1959, several groups focused on detecting underground nuclear tests were established within the US Department of Defense; on September 2, 1959, the Secretary of Defense assigned the research responsibilities to the Advanced Research Projects Agency (ARPA). This initial assignment involved seven tasks: creating a worldwide network of standardized seismological stations; conduct research relating to underground test detection, and on seismic detection stations; conduct nuclear and chemical underground tests to expand knowledge of seismic signatures; and three tasks of a more administrative nature. From the beginning, however, and through 1960, ARPA pressed to add research for testing surface-based, atmospheric, and outer-space detonations, and advocated satellites as a potential platform for such detections.

The Vela Project started as a small budget research program within the DARPA Projects Agencies until 1961 when it was granted greater funding and resources through the authority of the Deputy Assistant Secretary of the Army. This was likely prompted by increased caution over Domestic Nuclear Affairs resulting from the 1961 Goldsboro B-52 crash as well as in anticipation of the 1963 PTB Treaty.

The initial project was limited to the development of the Vela Satellites, but was later expanded and reorganized into three distinct levels of monitoring:

The project's space and atmospheric divisions operated until the emplacement of satellites in the Defense Support Program (DSP), and were later integrated into the Global Positioning System set of satellites. Currently, all satellites under Project Vela are under the management of IONDS. The seismic division of Project Vela remains in activity today.

In 2020, National Security Science magazine featured an article on Project Vela titled "Cold War watchmen" that contained a description of the project history focusing on the contributions made at Los Alamos National Laboratory, as well as commentary from scientists that worked on the project. In the article astrophysicist Ed Fenimore states "Vela was the prototypical project that made Los Alamos the premier scientific national security laboratory in the world." Data from Project Vela is still used for national security work, and sensors, such as those developed in the project, are still an important part of nuclear detonation detection.

There is very little documentation which directly addresses the topic of the initial Vela Project due to its low funding, staffing, resources and status of priority. The first sensor package sent into space, labelled the "Vela Hotel" experiment, was launched aboard the Ranger 1 lunar probe in August of 1961 and returned useful data. It is known that the majority of the work done on the initial Vela Project was implemented into Vela Hotel, which monitored outer space for nuclear tests. The Vela Hotel satellite program was primarily developed at Los Alamos Scientific Laboratory, under the supervision of the United States Air Force. A system of X-ray, Neutron, and Gamma-Ray detection monitored for nuclear testing beyond the atmosphere.

The satellites' instrumentation count included 12 X-ray detectors, 18 internal Neutron and Gamma-ray detectors, data transmission equipment to send the information back to Earth, and a set of solar panels which generated 90 watts to power the instrumentation on each satellite. Six of the Vela Hotel series satellites were manufactured, each with a design life of six months, and they were launched a week after the PTB Treaty on October 17, 1963. After deployment these satellites remained operational and online for 5 years before being shut down.