The crash happened on Friday in Nitshill, Glasgow, around 5.35pm near the Dunside Drive junction.
A grey 125cc M2R off-road bike ridden by the boy collided with a blue Mercedes C300 car.
READ MORE: 27-year-old hospitalised after being attacked in Dunoon
Emergency services attended and the teenager was taken to the Royal Hospital for Children with serious injuries.
His condition is described as serious but stable.
No one else was injured.
The road was closed for crash investigation work and re-opened around 7.35pm.
Sergeant Chris McColm said: “Our inquiries are ongoing to establish the full circumstances and we are keen to speak to anyone who saw what happened.
“We are also asking anyone driving in the area at the time to check any dashcam they have and get in touch if it shows something that could assist with our investigation.”
Anyone who can help is asked to contact Police Scotland on 101, quoting incident number 2063 of Friday, December 26 2025.