Observer (also known as Brain Guy) is a fictional character on the Mystery Science Theater 3000 television series. He was played by Bill Corbett, and appeared on the eighth through tenth seasons of the series. Observer is a hyper-intelligent, psychic alien from a planet of fellow aliens confusingly sharing the name "Observer". Supposedly, the Observers "evolved" beyond bodies into dark-green brains in large Petri dishes which are carried around by humanoid host bodies (controllable over a distance of up to 50 yards), rendering their abandonment of their original bodies rather pointless (As the robot Gypsy points out, "Wouldn't it be more convenient to just keep your brains in your heads?"). Thus Observer is, technically, only the brain which is being carried by the host body, but for all intent and purposes, he is considered a humanoid with brain separated from body. Observer joins the mad scientists ("The Mads") after his planet is inadvertently destroyed by Mike Nelson.

Like his colleagues, Professor Bobo and Pearl Forrester, Brain Guy is deeply dysfunctional. Unlike Bobo and Pearl, Brain Guy apparently has a considerable social and sexual life once the show returns to Earth for Seasons 9 and 10. He is also said to have, as Professor Bobo put it, "B.O." (body odor). Observer denies this, claiming that he doesn't have a body, although eventually he gives himself a sniff and admits that he does, in fact, "stink".

Observer, upon joining the crew of "Mads", usually ends up being the one who sends the movies to the Satellite of Love via his psychic abilities.

Observer claims, like his fellow Observers, to be omniscient and omnipotent, much like Q and his people from Star Trek: The Next Generation, but frequently fails to demonstrate these supposed abilities (he once stated that he was "not that omnipotent"). In one of his earlier appearances, before his homeworld is accidentally destroyed by Mike Nelson, his fellow Observers test the rest of the cast to see if any of them are deserving the right to become part of their kind and are surprised when Tom Servo scores higher than him, leading to Observer being painfully punished in Episode #806, "The Undead". Exposure to Pearl's autocratic manner also appears to degrade his powers over the course of the show, to the point when Observer tries to punish Mike horribly, he send him a necktie instead ("Don't you see what a terrible gift that is?") In the final episode, #1013 Danger: Diabolik, Pearl's playful dousing of his brain in Mountain Dew temporarily interferes with his speech and disables Brain Guy's gifts, allowing the Satellite of Love to crash to Earth.