Knockturn Alley is a shopping area off the south side of Diagon Alley.
From the Story
"Knockturn Alley is a dingy alleyway off Diagon Alley containing shops which sell items associated with the Dark Arts. Harry arrives here by accident when he fails to correctly pronounce ‘Diagon Alley’ while travelling by Floo powder."
- As of March 2013 it is locked, and it is not known when it will become available to enter.