Knockturn Alley, a pun on nocturnally, is a fictional location in J. K. Rowling's Harry Potter series of novels. Leading off of the more savoury Diagon Alley, Knockturn Alley is a dark and seedy alleyway in London to which muggles have no access, and where no respectable witch or wizard ought to be found either. All of the shops and stores in Knockturn Alley are devoted to the Dark Arts; the largest and most disreputable is Borgin & Burkes, which sells sinister and dangerous objects.

Despite its reputation, Hagrid visits Knockturn Alley in in search of Flesh-Eating Slug Repellent and other items which he presumably requires for the care and feeding of the exotic and dangerous creatures to which he tends to become attached.

Harry, while traveling via floo powder to Diagon Alley, accidentally landed in Knockturn Alley, where he spied on a conversation between Lucius Malfoy, his son and Mr. Burgin. Harry was discovered by a witch who clearly wanted to take advantage of him being lost and vulnerable, but he was rescued by Hagrid.