Starfleet is a fictional organization in the Star Trek media franchise. Within this fictional universe, Starfleet is a uniformed space force maintained by the United Federation of Planets ("the Federation") as the principal means for conducting deep space exploration, research, defense, peacekeeping, and diplomacy (although Starfleet predates the Federation, having originally been an Earth organization, as shown by the television series Star Trek: Enterprise). While most of Starfleet's members are human and it has been headquartered on Earth, hundreds of other species are also represented. Most of the franchise's protagonists are Starfleet commissioned officers.

During production of early episodes of the original series, several details of the makeup of the Star Trek universe had yet to be worked out, including the operating authority for the USS Enterprise. The terms Star Service ("The Conscience of the King"), Spacefleet Command ("The Squire of Gothos"), United Earth Space Probe Agency ("Charlie X" and "Tomorrow Is Yesterday"), and Space Central ("Miri") were all used to refer to the Enterprise's operating authority, before the term "Starfleet" became widespread from the episode "Court Martial" onwards.

However, references to the United Earth Space Probe Agency, and its abbreviation UESPA, are to be found in episodes of later series. For example, the Friendship One probe (launched, on the fictional timeline, in 2067) is marked with the letters UESPA-1 in the Star Trek: Voyager episode "Friendship One". Other background props included additional UESPA references, such as Captain Jean-Luc Picard's family album in Star Trek Generations. During the production of Star Trek: Enterprise, some larger Starfleet insignia designs included the name "United Earth Space Probe Agency".

Multiple Star Trek: Enterprise episodes refer to Starfleet having started operation some time between 2112 and 2136, when it funded research begun by Zefram Cochrane and Henry Archer, which led to the first successful flight of Warp-3 vessels in the 2140s. This research is said to have evolved into the NX Program, which led to Starfleet launching its first Warp 5–capable starship, Enterprise (NX-01), in 2151, followed by Columbia (NX-02), in 2155, as well as other vessels.

Starfleet acts under the Prime Directive, a policy of non-interference with pre-warp worlds, such as interference in their internal politics. This is said not to be a human construct, but stems from policies originally implemented by the Vulcans, who regarded an alien civilization's attainment of warp speed as the reason for making first contact with them. This was to avoid any unfortunate incidences during space travel, as well as avoiding interfering in the natural development of a civilization. The Prime Directive and Starfleet's first-contact policies are at the center of several episodes in each Star Trek series and the film Star Trek: First Contact.

Starfleet Headquarters is shown to be located on Earth, northeast of the Golden Gate Bridge in the present-day Fort Baker area. Starfleet Academy is located in the same general area. Additionally, various episodes show Starfleet operating a series of starbases throughout Federation territory, as ground facilities, or as space stations in planetary orbit or in deep space. One example is Deep Space Nine, a station near a wormhole commanded by Benjamin Sisko after its transfer from the Cardassian Empire.

Starfleet has been shown to handle scientific, defense, and diplomatic missions, although its primary mandate seems to be peaceful exploration in the search for sentient life, as seen in the mission statements of different incarnations of the USS Enterprise. The flagship of Starfleet is often considered to be the starship USS Enterprise.

Starfleet has many components, including: