The incident occurred on the N59 at Eskeragh, Crossmolina, near Ballina. Emergency services arrived to the scene at around 11.20am on Sunday morning.
The boy was pronounced deseased at the scene. His body has been taken to the mortuary at Mayo University Hospital.
Gardaí are appealing for informationa and for eye witnesses to come forward.
The scene is currently closed for forensic examination and traffic diversions have been put in place.
They have asked for road users with camera footage (including dash-cam) from the area between 10.40am and 11.40am to come forward.
Anyone with information is asked to contact Ballina Garda Station on 096 20560, the Garda Confidential Line on 1800 666 111, or any garda station.
