Torrential overnight rain made the hillside on the A490 near Guilsfield unstable and the landslide happened late afternoon.
Police warned people to stay away from the area as diversions were put in place.
The landslide, which took trees with it onto the main road happened near to the Valley View Holiday Park, Pentre’r Beirdd near Trefnanney.
More rain was expected overnight.
Also in mid Wales trains between Newtown and Machynlleth were cancelled because of flooding on the track.