It's understood Shaneen McCullough travelled to Helen's Bay from Portadown on Sunday, June 22 and did not return home.
The 15-year-old is said to be 5ft 6in tall, of heavy build with long brown/blonde hair.
She was last seen wearing a blue vest top, denim skirt and floral cardigan and was carrying a bag.
It is possible that Shaneen may be in the east Belfast area, police have said.
Anyone with information can contact police on 101.
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