A hay bale stacker apparatus (2) is disclosed including a trailer (4) having a chassis (6) and a hay storage platform (8). A hay bale clamping assembly (J) is positioned at a longitudinal edge of the platform (8) and includes a first clamping member (12) and a second clamping member substantially parallel to one another. The clamping members are substantially perpendicular to the ground surface in a hay bale gathering position. The clamping assembly is offset with respect to the platform (8) and is pivotally coupled to the platform (8) to selectively move between the horizontal hay bale gathering, clamping position and a vertical hay bale unloading position. When the hay bale clamping assembly (J) is in the vertical hay bale unloading position the first clamping member (12) is located above the second clamping member and is pivotally rotatable with respect to the second clamping member. Rotation of the first clamping member (12) moves the hay bale (4) off the second hay bale clamping member onto the adjacent platform (8).
