Reginald Endicott Barclay III is a fictional engineer from the Star Trek media franchise. On television and in film, he has been portrayed by Dwight Schultz since the character's introduction in the Star Trek: The Next Generation third season episode "Hollow Pursuits". Schultz played the character for five episodes and one feature film (Star Trek: First Contact) of The Next Generation, as well as six episodes of the series Star Trek: Voyager. Barclay also appears in non-canon Star Trek novels and video games.

Schultz was cast in The Next Generation thanks to Whoopi Goldberg (Guinan). Initially disappointed not to be playing an alien character, Schultz never anticipated becoming such a fan-favorite. Barclay underwent changes from, as first conceived, a malicious character—someone that aggravated the crew to compensate for his inferiority complex—to the softer and more benign engineer that made it to air. Schultz and others saw Barclay as a tribute to the average Star Trek fan, though the producers said that was unintentional.

During The Next Generation's televised run, Barclay was stationed aboard the USS Enterprise (NCC-1701-D). He was then posted at the Jupiter Station Holoprogramming Center before being aboard the Enterprise-E for the events of Star Trek: First Contact. For his season six and seven Star Trek: Voyager appearances, Barclay was assigned to the Pathfinder Project, a Starfleet Communications endeavor to communicate with the USS Voyager stranded in the Delta Quadrant.

University of Rochester professor Sarah Higley created Reginald Barclay in her script for the Star Trek: The Next Generation third-season episode "Hollow Pursuits". Before writing the script, Higley knew that she wanted to write about the less-exceptional members of the Enterprise crew, saying, "I thought there wasn't enough emphasis on the less outstanding, less wunderkind-like crew members." She described her initial conception of Barclay as "a repulsive guy […] wrapped up in himself, brilliant in his field, [and] socially a wash-out." She gave him the pompous name Endicott Reginald Barclay III to suggest he "had always suffered from an inferiority complex and just couldn't deal with the ultra-maturity and shining qualities of the rest of the crew." Higley based Barclay on Till Eulenspiegel, a prankster from German folklore: the original Barclay pranked the crew, taunted them about their weaknesses, and called their bluff before being sent to a psychiatric hospital.

Paramount bought "Hollow Pursuits" within a week of Higley's submission, but wanted to change the script to make Barclay save the day while being less acerbic and sarcastic. Higley would later recall how she had made Barclay "repugnant" while the studio wanted somebody with whom the audience could identify: someone "whose stuttering, shyness and lack of self-confidence would speak to [the] shy and socially maladjusted." As the character's actor, Dwight Schultz described Barclay as a ground-breaking character in terms of his normal fallibility. In the British magazine, TV Zone, Schultz thanked Roddenberry for creating the character, saying, "He realized that that type of character was missing from the programme and, eventually, said, 'Let's fill that spot.'"

In a script for "Hollow Pursuits", Barclay is described as a "shy individual with a perpetual frown born of stress and unhappiness between his brows. […] What we notice most about him is that he is uncomfortable around other people to the point of agony… he rarely makes eye contact… when he speaks, he hesitates, desperately afraid of saying the wrong thing… which he occasionally does."

Schultz was a fan of the original Star Trek; it was the first television series he watched in color. Schultz would wait for each episode to air, and then meet with friends after school to discuss the themes and plot of each one. The Next Generation showrunner Michael Piller recalled that sometime prior to casting Schultz, the actor had told executive producer Rick Berman that he would love to appear on the show. Schultz himself credits Whoopi Goldberg (Guinan) with his opportunity to play Barclay. In 1990, Schultz was working with Goldberg on The Long Walk Home when he discussed with her that he was a big fan of The Next Generation and knew Brent Spiner (Data) and Jonathan Frakes (William Riker) from his time in New York City. Then, approximately a month after returning from The Long Walk Home, Schultz heard from his talent manager that the powers-that-be at The Next Generation had extended a job offer. Piller and Berman did not think they would actually be able to land Schultz for the spot because the actor had done Fat Man and Little Boy with Paul Newman and "was very hot". Schultz anticipated playing "a large vein-throbbing, bulbous-headed alien," and was initially disappointed to be offered Barclay instead. Schultz thought Barclay was going to be merely a one-off character, and did not anticipate it becoming a recurring gig.

In the Star Trek canon, Barclay has appeared in twelve discrete productions from 1990 to 2001.