He was tried behind closed doors and convicted on a charge of theft with violence causing death.
The now 18-year-old was identified as the main person who triggered the woman's fall along the road south of Noumea.
The victim was a 31-year-old nurse going home after a nightshift.
The group of four teenagers was found to have wanted to steal her motorbike but fled when other people came near the scene.
The accident in March last year initially triggered an intense debate on social media over whether her fall was the result of foul play.
Three months later, the four teenagers - the youngest of them 14 - confessed to planning a car-jacking but then decided to seize the motorcycle instead as they heard it approach.
Two of the other teenagers were earlier jailed for six and seven years respectively.