Rebecca King, 24, was last seen on the 700 bus in Wallace Avenue, Goring, at 3.30pm today.
Inspector Kris Ottery said: “Rebecca has learning difficulties and behaves much younger than her actual age.
“We believe she could be somewhere along the seafront, probably in West Sussex. If you see her, please tell us urgently.”
She is described as 5ft 10ins tall, slim with wavy, mousy hair.
She is believed to be wearing jeans, a brown coat and grey trainers.
If you see Rebecca or know where she could be, call 101 or email [email protected] quoting serial 972 of 19/05.
If Rebecca appears to be in danger or hurt call 999 immediately.