The collision, on the A470 Llanidloes bypass blocked the road between the town and Llangurig.
Emergency services were called to the scene at about 3pm on Friday.
Traffic Wales said the A470 was closed, with emergency services on scene.
Dyfed Police tweeted: "There are currently serious delays on the the A470 between Llangurig and Llanidloes, due to a collision. Please be patient and take an alternative route if possible, or plan extra time into your journey."
The road was reported as open again shortly after 7.20pm.