Schoolgirl Rebecca McLean, was last seen on Saturday morning in Ryde on the Isle of Wight with 33-year-old Michael Britten, Hampshire police said.
The pair, who are both from Southampton, are believed to have travelled to the island off the south coast together.
Police said they "are growing concerned for their welfare".
They are urging anyone with information about Rebecca or Mr Britten to contact Southampton police on 101.
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