The security service of Starfleet, responsible for protecting its personnel and facilities, including internal security on Earth.
Starfleet Security protects Starfleet installations and personnel and supplies security officers to ships and stations. Michael Eddington is assigned to Deep Space Nine as a Starfleet Security officer before he reveals himself as a member of the Maquis.
In Homefront, after a Changeling bombing in Antwerp, Benjamin Sisko becomes acting head of Starfleet Security on Earth. In Paradise Lost he discovers that Admiral Leyton is exploiting fears of infiltration to stage a military coup, and he stops it before Earth falls under martial law.