Megan Stammers, from Eastbourne, was reported missing after she failed to turn up for school on Friday and was later spotted using a Dover to Calais ferry.
Officers believe she is travelling with a man she knows - Jeremy Forrest, from Ringmer, near Lewes - and have appealed for the pair to get in touch.
Officers are looking into the whereabouts of his black Ford Focus, registration GJ08 RJO.
Megan Stammers' father has made an appeal for her to get in touch after she went missing from her home in Eastbourne, East Sussex, yesterday and fled across the English Channel with Mr Forrest.
Her father, Martin Stammers, said: "We just want Megan to make contact with us. We are worried and miss her terribly. Please get in touch, Megan."
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Sussex Police say they are in contact with the French authorities in an effort to reunite her with relatives.
Detective Inspector Andy Harbour, of East Sussex CID, said she travelled from Dover to Calais at around 9.20pm yesterday - nine hours after she was reported missing to police.
Mr Harbour said: "We do not believe Megan to be at risk but we are appealing for her to make contact with her family who are very worried and miss her.
"Megan was reported missing to us at 12.30pm yesterday after she failed to turn up for school.
'We know that she crossed from Dover to Calais at around time 9.20 yesterday evening. We believe she is still in France and we are in contact with the French authorities."
Chief Inspector Jason Tingley said: "Megan's family love her dearly and are all incredibly upset by her disappearance.
"They want her to come home as soon as possible. We need Megan or Jeremy to make contact with us to help us return her safely to her family."
Megan is described as 5ft 6in tall, with long, dark brown hair and was last seen wearing a white vest and a silver necklace.
Anyone with information is asked to call officers at Sussex Police.