Police are concerned for Bonnie Rose May Clarke who has gone missing from Southwick.
The 17-year-old had been visiting family in Southwick last Friday but did not return to where she had been staying and was reported the next day.
Bonnie lives in Wales but knows Sussex very well and is a frequent visitor to Brighton, Worthing and Eastbourne, police say.
She is white, 5ft 9ins, with shoulder length curly brown hair and a Welsh accent.
She was last seen wearing a black and blue floral maxi dress and baby blue hooded top.
Anyone who sees her is asked to contact police online or phone 101 quoting serial 541 of 01/07.