The paramilitary style shooting took place on Tuesday at around 8.30pm in the Ardoyne Avenue area.
The 18-year-old was shot a number of times in the thigh and buttock and is being treated at the Royal Victoria Hospital in the city.
A spokesperson for the Belfast Trust described his condition as critical on Wednesday.
A 39-year-old man has been arrested in relation to the attack.
Police want to hear from anyone who saw or heard anything suspicious, particularly the movements of vehicles, around the Flax Centre off Ardoyne Avenue between 8-9pm on Tuesday night.
Oldpark councillor Nichola Mallon condemned those responsible.
"I understand the young victim of this apparent paramilitary style shooting is in a critical condition," said the SDLP representative. "Our thoughts are with him and his family.
"This was a horrific and unjustifiable attack.
"Kangaroo courts and vigilante justice is not welcome in Ardoyne and must not be allowed to have any place in our society."