Officers were called shortly after 10.30pm after the collision which occurred on the A36 near Codford.
The pedestrian, a man in his 40s, was taken to Southmead Hospital in Bristol where he remains in a life-threatening condition.
No arrests have been made at this time.
The road remains closed at Heytesbury and Deptford and is likely to remain closed for most of the morning.
We would ask motorists to use alternative routes.
Anyone who witnessed the collision is asked to contact us now on 101 and quote log number 343 of July 31st.
