Dawn Summers is a fictional character created by Joss Whedon and introduced by Marti Noxon and David Fury on the television series Buffy the Vampire Slayer, portrayed by Michelle Trachtenberg. She made her debut in the premiere episode of the show's fifth season, and subsequently appeared in every episode of its remaining three seasons. Within the series, Dawn is the little sister of main character Buffy Summers (Sarah Michelle Gellar), a girl chosen by fate to be a vampire Slayer. Whedon introduced Dawn to the series because he wanted to introduce a character with whom Buffy could have an intensely emotional non-romantic relationship.

After years of foreshadowing, Dawn was introduced at the start of the fifth season as part of a large in-story retcon: characters accepted Dawn's presence as if she had always been there, and as if Buffy always had a sister, with only the audience aware that this was not the case. As the series went on, the significance of Dawn's arrival is revealed to the series' other characters and they come to understand that she has not always been Buffy's sister, or indeed a sentient being; Dawn had originally been the mystical 'key' to unlocking dimensions, and was made into Buffy's sister so the Slayer would protect her. Dawn is, however, a real girl, Buffy's biological sister, and has real memories of her fictional childhood. She struggles in a very human way when she discovers the truth about her origins, and later endures problems with self-harm and kleptomania. The show's sixth and seventh seasons, as well as its canonical comic book continuation, follow Dawn's journey towards adulthood.