Starshield is a business unit of SpaceX creating purpose-built low-Earth-orbit (LEO) satellites designed to provide new military space capabilities to U.S. and allied governments. Starshield was adapted from the global communications network Starlink but brings additional capabilities such as target tracking, optical and radio reconnaissance, and early missile warning. Primary customers include the Space Development Agency (SDA), National Reconnaissance Office and the United States Space Force. As of 2025, at least 183 Starshield satellites have been launched, with the latest batch of 22 satellites being launched in April 2025 as part of NROL-145.

While SpaceX president and COO Gwynne Shotwell has indicated that there is little information she is allowed to disclose about Starshield, she has noted "very good collaboration" between the intelligence community and SpaceX on the program. The U.S. Congressional Research Service reports that future satellites in Starshield's participating SDA program may wield interceptor missiles, hypersonic projectiles, or directed energy weapons, with the program's founder adding "since Reagan’s day, technology has advanced enough that putting both sensors and shooters in space is not only possible but relatively easy." According to SDA director Derek Tournear, later satellites will take on the “extremely difficult” task of maintaining contact with missiles in flight.

The former four-star general Terrence O'Shaughnessy, who previously ran U.S. Northern Command, is the vice president for SpaceX's Special Programs Group who is thought to be involved with Starshield. The Wall Street Journal reported that Starshield's online job postings required people with top-secret clearances, as well as experience working with the Defense Department and intelligence community — such as representing Starshield to Pentagon combatant commands. For weapons manufacturing, eight senior Starshield leaders formed an additional company Castelion, to develop mass produced hypersonic strike weapons, potentially for use as space-based interceptors

The first satellites were designed for the SDA and outfitted with advanced infrared sensors meant to detect and track ballistic and hypersonic missiles. In 2021, Starshield had entered a $1.8 billion classified contract with the U.S. government, revealed in 2023, to construct hundreds of spy satellites for continuous real-time monitoring of targets around the globe. These began operations from May 2024, starting with NROL-146. These satellites are made in cooperation with Northrop Grumman.

The Starshield name was publicly announced December 2022, however in 2021, Starshield had already entered a $1.8 billion classified contract with the U.S. government, revealed in 2023. In the documents of the contract, SpaceX says that funds from the contract were expected to become an important part of the revenue mix of the company after 2021. Reuters revealed in 2024 that this contract was between the National Reconnaissance Office and SpaceX, and for a spy satellite network consisting of hundreds of satellites functioning as a swarm. The satellites will have imaging capabilities, and the satellite network will enable the US government to have continuous surveillance of nearly anywhere around the globe. Starshield also plans to be more resilient to attack from other powers. Starshield's imaging capabilities are designed to have superior resolution over most existing U.S. government spying systems. Northrop Grumman was selected to partner with SpaceX, with insiders noting that "it is in the government's interest to not be totally invested in one company run by one person".

As early as 2020, SpaceX was designing, building, and launching customized satellites based on variants of the Starlink satellite bus for the National Reconnaissance Office (NRO).

In October 2020, SDA awarded SpaceX an initial $150 million dual-use contract to develop 4 satellites to detect and track ballistic and hypersonic missiles. The first batch of satellites were originally scheduled to launch September 2022 to form part of the Tracking Layer Tranche 0 of the Space Force's National Defense Space Architecture. The launch was rescheduled multiple times but it eventually launched in April 2023.

In 2020, SpaceX hired retired four-star general Terrence J. O'Shaughnessy who according to some sources is associated with Starlink's military satellite development and according to one source is listed as a "chief operating officer" at SpaceX. While still in active duty, O'Shaughnessy advocated before the United States Senate Committee on Armed Services for a layered capability with lethal follow-on that incorporates machine learning and artificial intelligence to gather and act upon sensor data quickly. As of 2024, Terrence O’Shaughnessy reportedly has had a high-level role at Starshield, with SpaceX reportedly vying for contracts for the Golden Dome space weapons system.