A man in his 40’s was airlifted to Mayo University Hospital after the van he was driving collided with a car near Belmullet.
The man is described as being in a serious condition.
The occupants of the car included three females and a baby.
They were also taken to hospital and sustained non-life threatening injuries.
The collision on the R313 at Muinhin Bridge, which is between Bangor and Belmullet occurred at 3.15pm.
The road will remain closed overnight for a forensic examination to take place.
Diversions are in place.