Secondary payload, also known as rideshare payload, is a smaller-sized payload transported to orbit on a launch vehicle that is mostly paid for—and with the date and time of launch and the orbital trajectory determined—by the entity that contracts and pays for the primary launch. As a result, the secondary payload typically obtains a substantially reduced price for transportation services to orbit, by accepting a trade off of the loss of control once the contract is signed and the payload is delivered to the launch vehicle supplier for integration to the launch vehicle. These tradeoffs typically include having little or no control over the launch date/time, the final orbital parameters, or the ability to halt the launch and remove the payload should a payload failure occur during ground processing prior to launch, as the primary payload typically purchases all of these launch property rights via contract with the launch services provider.

While originally a US government-centric option for government-owned launches—where secondary payload slots were often given away by whatever allocation means a government agency might choose—an entire market has emerged over time to take advantage of the lower cost of access to space through secondary payload opportunities.

The small satellite segment of the satellite launch industry has been growing rapidly in recent years. Development activity has been particularly high in the 1–50 kg size range. In the 1–50 kg range alone, there were fewer than 15 satellites launched annually in 2000 to 2005, 34 in 2006, then fewer than 30 launches annually during 2007 to 2011. This rose to 34 launched in 2012, and 92 small satellites launched in 2013. In 2023, 2,304 small satellites were launched, an 18% increase from 2022

Offering of secondary launch services vary by launch provider. US commercial launcher United Launch Alliance (ULA) offers virtually no access for secondary payloads commercially, although the US military offers some secondary payload slots on ULA launchers Atlas V and Delta IV, that are then controlled by government launch slot allocation processes.

Rocket Lab offers rideshare capabilities in their existing Electron rocket. With the ability of the Kick Stage's ability to reignite, the vehicle can alternate between various orbits to deploy the various payloads.

The Photon is a satellite bus designed by Rocket Lab that is a major enhancement to the Kick Stage. It is able to launch multiple satellites to low Earth orbit (LEO), medium Earth orbit (MEO), geostationary orbit (GEO), lunar, and planetary destinations.

SpaceX offered a priced set of secondary payload launches on their Falcon 9 rocket beginning in 2011 with prices between US$200,000–325,000 for secondary payloads delivered to low Earth orbit (LEO). As of March 2014^[update] SpaceX indicated that they would continue to launch some secondary payloads, but would not be doing a lot of them, as there was "still not a lot of money in the secondary payload market".

In early August 2019, SpaceX announced a rideshare program for launching small satellites into orbit when their large satellite market was shrinking after 2018. Although SpaceX had previously flown a dedicated secondary payload mission, the program would make customers buy ports directly from SpaceX. Initially SpaceX offered to launch secondary payloads up to 150 kg (330 lb) to a Sun-synchronous orbit for US$2.25 million if customers signed up at least 12 months before the launch. If between 6 and 12 months, prices would be increased to US$3 million. For secondary payloads up to 300 kg (660 lb) if customers signed up 12 months in advance SpaceX offered a base price of US$4.5 million and if 6 months in advance US$6 million. Flights were planned to be launched from SLC-4E at Vandenberg Air Force Base, starting from November 2020 on the Falcon 9 rocket.