A male passer-by entered the Shing Mun River and rescued a 15-year-old boy who had fallen from a bridge near Shek Mun Estate in Sha Tin at about 5pm on October 5.
Emergency personnel found the boy conscious after the rescue, and he was taken to Prince of Wales Hospital for treatment.
Police provisionally classified the case as attempted suicide and were continuing to investigate what led to the incident.
The Emotional Support Hotline and WhatsApp service is available on 18111, the Samaritans Prevention of Suicide hotline on 2389 2222, and the Social Welfare Department hotline on 2343 2255.