Leanne Walker was last seen by her family in the Glusburn area of Craven at midnight on Wednesday.
North Yorkshire Police said the 36-year-old is in a "distressed and vulnerable state" and the force has made an urgent appeal for information as to her whereabouts.
A force spokesperson said: "Extensive police enquiries have been going on in the area overnight, and a full search is being conducted today with the use of specialist officers and search and rescue teams.
"To help the effort to find Leanne, members of the public are urged to keep an eye out for her and report any information to the police without delay."
She is described as white, 5ft 5ins, slim, with long dark-coloured hair. She was last seen wearing a light denim jacket with a white shirt underneath, white jeans and Converse-style trainers.
Anyone with information is asked to contact North Yorkshire Police on 101, quoting reference number 12160172540.