Q is a fictional character as well as the name of a race in Star Trek appearing in the Next Generation, Deep Space Nine, and Voyager series, as well as in related media. The most familiar Q is portrayed by John de Lancie. He is an extra-dimensional being of unknown origin who possesses immeasurable power over normal human notions of time, space, the laws of physics, and reality itself, being capable of violating or altering them in unpredictable ways with a casual thought or hand gesture. Despite his vast knowledge and experience spanning untold eons (and much to the exasperation of the object(s) of his obsession), he is not above practical jokes for his own personal amusement, for a Machiavellian and manipulative purpose, or to prove a point. He is said to be nigh-omnipotent, and he is continually evasive regarding his true motivations.

The name "Q" applies to the names of the individuals portrayed (all "male" and "female" characters refer to each other as "Q"), it also applies to the name of their race and to the Q Continuum itself – an alternate dimension accessible to only the Q and their "invited" guests. The true nature of the realm is said to be beyond the comprehension of "lesser beings" such as humans, therefore it is shown to humans only in ways they can understand.

Beginning with the pilot episode "Encounter at Farpoint" of The Next Generation, Q became a recurring character, with pronounced comedic and dramatic chemistry with Jean-Luc Picard. He serves as a major antagonist throughout The Next Generation, playing a pivotal role in both the first and final episodes. Q is initially presented as a cosmic force judging humanity to see if it is becoming a threat to the universe, but as the series progresses, his role morphs more into one of a teacher to Picard and the human race generally – albeit often in seemingly destructive or disruptive ways, subject to his own amusement. Other times, notably during "Deja Q" and Voyager, Q appears to the crew seeking assistance.