MCCOY: He’s dead, Jim.

Richard Matheson (1926-2013) American author and screenwriter

Star Trek, 1×05 “The Enemy Within” (1966-10-06)
    (Source)

First use of the phrase that became a trademark for DeForest Kelley's Dr. Leonard "Bones" McCoy. In this first instance, it's applied to an alien animal that has been run through the malfunctioning transporter to (lethally) re-integrate its "good" and "evil" halves.

Matheson did the initial screenplay and multiple revisions, and gets the writing credit for the episode, but John Black and Gene Roddenberry also "polished" the script, so the precise provenance of the line which, with variations, showed up in multiple subsequent episodes, is unknown.