The Homicide and Major Enquiry team (HMET) has launched an investigation after the incident in Grosvenor Park on Lumb Lane, Manningham, on Saturday (March 11).
He was found with a stab wound to his chest.
Two teenagers were arrested in connection with the incident.
One 15-year-old has been bailed pending further enquiries, and another 15-year-old remains in police custody at this time on suspicion of attempted murder.
Detective Chief Inspector Alan Weekes, of HMET, said: “This is a very serious incident where a 16-year-old boy has been stabbed and remains critically ill in hospital.
“My thoughts are with the victim and his family who, as you can imagine, are distraught.
“We know there were numerous young people in the park between 4.30pm and 5.30pm on Saturday who may have witnessed the disorder.
“I would appeal directly to each one of them to come forward and provide evidence in the investigation.
“We are determined to identify those responsible for this incident and bring them to justice.
“We know that people who were there may have reservations about speaking with the police, but we hope that they can appreciate that this is important for the victim and the investigation.”
Chief Inspector Bash Anwar, of Bradford District Police, added: “This incident has shocked the local community. My neighbourhood officers are providing support and reassurance.
“We are appealing directly to the people who might have seen what happened to come forward as they may have information that will help the investigation.
“Our colleagues at HMET need the support of the community and I would urge anyone who has any information, however small, to assist.
“We recognise that an incident like this will cause understandable concern in the community, and we need the community to take a stand against this type of violence.
“Knives have no place on our streets.”
Anyone who has any information that could help the ongoing police investigation is asked to contact HMET by using 101 Live Chat online or by calling 101, quoting crime reference number 13230140005.
Information can also be passed anonymously to independent charity Crimestoppers on 0800 555 111.