Hannah Mackenzie, 17, from Gravir was last seen at about 4pm on Thursday (16 August) outside the Nicolson Institute in Stornoway and was reported missing later that evening.
She is described as being 5ft 5in tall, slim build, with dark hair worn in a ponytail.
She was last seen a dark blue Nicolson Institute sweater, wearing black skinny jeans and a black waist length jacket with white lining and white furry strip on the hood.
Hebrides Search and HM Coastguard have joined Police Scotland officers in the search for Hannah, with particular focus on the Castle Grounds in Stornoway following a reported sighting in the area on Thursday evening.
Inspector Jane Nicolson said: “We are grateful to everyone who has been in contact so far with information and encourage anyone else who can help to get in touch.
“If Hannah herself is reading this then please get in touch and let us and your family know you are safe and well.
“You can call Stornoway Police Station with information by calling 101, quoting reference NH1731/18.”