Abstract: A guide device for boat trailers includes a pair of guide devices each being mounted on one of the rear frame members of the boat trailer and each guide device including a vertically oriented guide roller. Each guide device also includes an obliquely oriented roller which is positioned adjacent the associated vertical roller. The vertical rollers are vertically adjustable and each oblique roller and its associated vertical roller are laterally adjustable as a unit to permit proper spacing between the guide devices to maintain a boat in centered relation as it is pulled upon the trailer.