A Trekkie or Trekker is a fan of the Star Trek franchise, or of specific television series or films within that franchise. The show developed a following shortly after it premiered, with the first fanzine premiering in 1967. The first fan convention took place the year the original series ended.

The degree of Trekkies' devotion has produced conflicted feelings among the cast and crew of the show. Creator Gene Roddenberry initially encouraged the fan participation, but over the years became concerned that some fans treated the show with a quasi-religious zeal as though it were "scripture." While some stars have been vocally critical of the franchise's most devoted fans, others including Sir Patrick Stewart have defended Trekkies.

There has been some disagreement within the fandom as to the distinction between the terms "Trekker" and "Trekkie." Some characterize Trekkers are "more serious" in comparison to the "bubble-headed" Trekkies, while others have chosen the term Trekker to convey that they are "a rational fan." Leonard Nimoy advocated for the use of "Trekker" over "Trekkie". Overall, the term "Trekkie" is more commonly used.

Many early Trekkies were also fans of The Man from U.N.C.L.E. (1964–1968), another show with science fiction elements and a devoted audience. The first Star Trek fanzine, Spockanalia, appeared in September 1967, including the first published fan fiction based on the show. Star Trek creator Gene Roddenberry, who was aware of and encouraged such activities, a year later estimated that 10,000 wrote or read fanzines. The mainstream science fiction magazine If published a poem about the Star Trek character Spock, accompanying a Virgil Finlay portrait of the character.

Perhaps the first large gathering of fans occurred in April 1967. When actor Leonard Nimoy appeared as Spock as grand marshal of the Medford Pear Blossom Festival parade in Oregon, he hoped to sign hundreds of autographs but thousands of people appeared; after being rescued by police, "I made sure never to appear publicly again in Vulcan guise", the actor wrote. Another was in January 1968, when more than 200 Caltech students marched to NBC's Burbank, California studio to support Star Trek's renewal.

The first fan convention devoted to the show occurred on 1 March 1969 at the Newark Public Library. Organized by a librarian who was one of the creators of Spockanalia, the "Star Trek Con" did not have celebrity guests but did have "slide shows of 'Trek' aliens, skits and a fan panel to discuss 'The Star Trek Phenomenon.'" Some fans were so devoted that they complained to a Canadian TV station when it preempted an episode in July 1969 for coverage of Apollo 11.

Nothing fades faster than a canceled television series they say. So how come Star Trek won't go away?

However, the Trekkie phenomenon did not come to the attention of the general public until after the show was cancelled in 1969 and reruns entered syndication. The first widely publicized fan convention occurred in January 1972 at the Statler Hilton Hotel in New York, featuring Roddenberry, Isaac Asimov, and two tons of NASA memorabilia. The organizers expected 500 attendees at the "First International Star Trek Convention" but more than 3,000 came; attendees later described it as "packed" and like "a rush-hour subway train". By then more than 100 fanzines about the show existed, its reruns were syndicated to 125 American TV stations and 60 other countries, and news reports on the convention caused other fans, who had believed themselves to be alone, to organize.