Abstract: A round bale trailer includes a trailer that travels in a hay field in the same direction that the field is normally worked to approach round hay bales in a direction perpendicular to the bales' axes. A U-shaped fork on the round bale trailer engages three sides of the bale. A closing arm on the fork closes to engage the bale on the fourth side. Continued motion of the trailer causes the fork and bale to rotate together 90 degrees about a vertical axis such that the fork closes to nest within a generally U-shaped frame. A hydraulic cylinder immediately operates to pivot the frame and fork to a vertical attitude, such that the bale rolls off the fork and onto the trailer. Pivoting the frame to the horizontal attitude automatically opens the fork by the use of a spring. All the bales are unloaded at one time by an unloading mechanism that tilts the trailer such that the bales roll off one side thereof.