Piia Irmeli Ristikankare (born 18 December 1972^[1]) was a 15-year-old girl from Piikkiö, Finland who disappeared on 7 October 1988.^[2] Her disappearance is among the most famous unresolved disappearance cases in Finland.^[3]

7 October was a normal Friday school day for Ristikankare, who was studying housekeeping at a vocational school.^[4] She had arranged a sleepover with a friend in Paimio, but had to cancel it because her 3-year-old brother needed a babysitter that night. Piia's parents were divorced and her mother had moved out of the family home a couple of years earlier.^[5]

On the night of the disappearance, the family's father was in the sauna with his father-in-law while Piia Ristikankare was spending time with her brothers, aged 3 and 14. The teenagers got into an argument over television programmes and Piia went out, probably taking a small bag with some money in it with her. She has not been seen since.^[1][6]

Several clues to the case have been reported and false sightings of Piia Ristikankare were also made after her disappearance.^[7][8] The case was taken over by the National Bureau of Investigation in 2003. According to a childhood friend, Piia had been bullied at school because of the family's religious orientation. The Ristikankare family are Jehovah's Witnesses.^[9] They continued living in the house from which Piia Ristikankare disappeared after the disappearance, despite several police raids.^[10]

Piia Ristikankare was officially declared deceased in absentia in 2011. The date of her death was marked as 1 January 2011.^[5]

• List of people who disappeared