Crews from Newtown and Welshpool were called to extinguish the blaze, which broke out at 9.39am on the B4385 between Montgomery and Abermule.
The trailer was carrying approximately 30 tonnes of straw when it caught fire.
A spokesman for Mid and West Wales Fire and Rescue Service said: "There was some disruption, police were informed because of the fire and it being a large trailer.
"They assisted with traffic during the incident."
The scene was cleared by 1.36pm.