Q is a fictional character, as well as the name of a race, in the Star Trek franchise, appearing in the series Next Generation, Deep Space Nine, Voyager, Lower Decks, Picard, and Strange New Worlds 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 time, space, the laws of physics, and reality itself, being capable of altering it to his whim. Despite his vast knowledge and experience spanning untold eons, he is not above practical jokes for his personal amusement, a Machiavellian or manipulative purpose, or to prove a point. He is said to be almost completely omnipotent and 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; e.g., a run-down gas station in the "middle of nowhere".

Beginning with the pilot episode of The Next Generation, "Encounter at Farpoint", 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.

Gene Roddenberry chose the letter "Q" in honor of his friend Janet Quarton.

Many Star Trek television episodes and novels have featured Q and often have titles that play on the letter "Q".

Q debuted in "Encounter at Farpoint", where he puts Captain Picard and the Enterprise crew on trial, arguing that humanity is a dangerous race and should be destroyed. When they later save the life of a kidnapped alien, Q agrees to defer judgment, though he hints that it will not be the last time the crew sees him.

In "Hide and Q", he forces the Enterprise crew to participate in a war game against monsters he summoned, then makes a wager with Picard. He grants Commander William Riker the powers of a Q, then promises that if Riker rejects the powers, then Q will leave them alone. Riker uses the powers to save his friends and starts to lose himself to them, but ultimately manages to reject them. Q attempts to go back on his word, but the Q Continuum forcibly recalls him.

In "Q Who", he offers to divest himself of his powers and guide humanity through uncharted regions and prepare it for unknown threats. Picard argues that Q's services are unneeded (and unwanted), and Q rebuts him by teleporting the USS Enterprise to a distant system for their first encounter with the Borg. Unable to resist the Borg, Picard must ask Q to save the ship. Q returns the Enterprise home and tells Picard that other men would rather have died than ask for help. This episode also reveals Q has a past history and hostile relationship with Guinan.