The man climbed onto the lorry on Kirby Misperton Road, near the junction with the A169, at 9.50am.
Police have put a contraflow system in place, so traffic is able to pass the lorry. Diversion signs are being put in place.
However, the man has told officers he plans to stay there until he is removed, so motorists should expect delays in the area.
Motorists are urged to avoid the area.