The six-year-old was pulled unconscious from the bottom of the pool at an Angus leisure park after getting into difficulties.
He was found shortly before 3pm on Saturday afternoon at the Red Lion Caravan Park's leisure centre in Arbroath.
Tayside Police received a call from the Scottish Ambulance Service while paramedics were attending to the child. The boy, who is understood to be local to the area, was resuscitated at the scene and then taken to Ninewells Hospital in Dundee.
Tayside Police said that the child's condition was critical on Saturday night and reported no change in his condition last night. The boy is understood to have been swimming with his mother, sister and another child when he got into difficulties.
Police officers are working with officials from Angus Council to establish what happened, but there are not thought to be any suspicious circumstances. The leisure facility is privately owned.
Nicole McCormack, a member of staff at the caravan park, said: "There was a boy in the swimming pool with his mother and sister and another boy, whom we believe to be his brother.
"It seems he suffered some kind of medical episode and was underwater for a short while."
A statement from Tayside Police said: "A six-year-old child remains in a critical condition at Ninewells Hospital, Dundee after an incident at the Red Lion Caravan Park, Arbroath.
"About 3pm on Saturday, Tayside Police received a call from the Scottish Ambulance Service who were attending a report of a child having got into difficulties in the swimming pool.
"On arrival the emergency services found the young boy unconscious and he was attended to at the scene by paramedics. He was conveyed by ambulance under police escort to Ninewells Hospital where his condition is currently described as critical. Tayside Police is working with officers from Angus Council to establish the full circumstances."