North Yorkshire Police said the seven year old (not six-year-old as previously believed) was airlifted to Leeds General Infirmary with head injuries following the incident at around noon yesterday.
His condition is now described as critical.
In a statement, updated this morning, Lightwater Valley said: "We have been informed by North Yorkshire Police that the medical condition of the child involved in the incident at the park yesterday has deteriorated overnight and he is now in a critical condition.
MORE: Boy falls from Lightwater Valley rollercoaster - here's everything we know so far"We are devastated by this news and our thoughts are with the family.
"While the Health and Safety Executive (HSE) investigation is continuing, we will support them and be guided by their advice."
A joint investigation has been launched by North Yorkshire Police and The Health and Safety Executive. The HSE currently has primacy.
The ride, Twister, that the boy fell from, was closed yesterday although the rest of the theme park remained open.
An onlooker said the youngster was riding the Twister rollercoaster when he slipped out of the carriage and plunged between 20ft and 30ft.