Number One, in "The Cage", the original pilot episode of the science fiction television series Star Trek, was the un-named intellectual, problem-solving second-in-command serving under Captain Christopher Pike. Played by Majel Barrett, later the wife of Gene Roddenberry (who also went on to play Nurse Christine Chapel in the original Star Trek and Lwaxana Troi in Star Trek: The Next Generation), the character appears only in the unaired pilot. The prominence of a women among the crew of a starship was one of the reasons that the original Star Trek pilot was rejected by television networks. During "The Cage," she proves to her alien captors that humans would rather die than be slaves. Aspects of her character -- specifically, her cool demeanor and logical nature -- were merged with that of Spock (who does appear in "The Cage") during the regular run of the series.

In Star Trek: The Next Generation, Captain Jean-Luc Picard frequently uses the title "Number One" to refer to his first officer, William T. Riker, in a reference to the original Number One.