The 17-year-old pedestrian was rushed to Craigavon Area Hospital after being struck by a vehicle on the Avenue Road in Lurgan at its junction with Demesne Avenue.
The incident took place around 11.30pm close to the main entrance of Lurgan Park.
Although initially being described as “very seriously” injured, the teenager was later transferred to the Royal Victoria where his condition was reported to have worsened.
A PSNI spokesperson said: “At approximately 11.30pm, a pedestrian was in collision with a car.
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“The pedestrian, a 17-year-old male, was taken to hospital for treatment to his injuries which are currently being closely monitored but are thought to be very serious at this time.”
Inspector Ray Shaw said: “I would ask anyone who witnessed this incident to contact Lurgan police station on the non-emergency number 101.”