The Chief of Starfleet Security was a Starfleet flag officer who was responsible for overseeing the activities of Starfleet Security in defending the Federation, in accordance with Starfleet Intelligence.
Admiral Yves Gervais was Chief of Starfleet Security from 2271 to 2295, when he was assassinated by the Albino. (Orion Press)
In the 2360s the post was held by Vice Admiral Thomas Henry, followed by Vice Admiral Raner for a brief period in 2370. (TNG: "The Drumhead", "The Pegasus")
In the 2380s, Vice Admiral Albert Daltrey held the post. (Star Trek: Remington)
Lieutenant General Jovan Pendleton, Jr. was the chief in the mid-2380s. (Starfleet Command (RPG))
In 2406, Admiral Stephen Alcott gave an unspoken threat to complain to the Chief of Starfleet Security in regards to a mole hunt conducted by Commander Simeon Kozhukhov of Starfleet Internal Affairs. (The War of the Masters: "For We Should Grow Too Fond of It, Part I")