Adam Toovey, who also goes by the surname Weaving, was last spotted at 8am today at Queen Alexandra Hospital in Cosham.
The 21-year-old from Horndean has not been seen since.
Adam is white, 5ft 11in and slim. He was last seen wearing a green T-shirt with black and white sleeves, a black hoodie, black trousers and white trainers.
Officers are concerned for his welfare and are asking anyone who has seen Adam – or anyone who knows where he is – to call 101.