The present invention provides a door assembly for a walk-in bathtub. The assembly includes a frame shaped to fit a door threshold in the side of the walk-in bathtub, wherein the frame includes a track dedicated to accommodating a gasket along the length of the frame. The frame also includes a track dedicated to accommodating door hinges. In the preferred embodiment of the invention, the frame is made of a single extruded piece of aluminum that is powder coated to match the acrylic surface of the walk-in tub. The door is mounted onto the frame with hinges. A gasket is inserted into the track in the door frame and forms a water tight seal between the door and the frame. The door assembly can be installed in the bathtub as a separate, pre-assembled system, similar to a pre-hung door.