A man in his 20s is dead after a crash near Blenheim this morning.

A St John Ambulance spokesman said a road crew found the car on State Highway 1, about 20km south of the Marlborough town, about 11am.

"There was one person in the car, a male in his 20s, and I understand he was deceased at the scene," the spokesman said.

Police were at the scene, a spokeswoman said.