A sink fastener for fastening a sink unit with an outer flange within an opening in a countertop is provided. The sink fastener includes a frame for attachment to a bowl wall of the sink unit, a threaded vertical bolt rotatably mounted within the frame, a claw member having a lateral portion threadably mounted on the vertical bolt and an upstanding retainer portion, and a guide member for constraining the movement of the claw member. In use, rotation of the vertical bolt causes the claw member to rise up the bolt to engage an underside of the countertop, and in a lower disengaged position the claw member is free to turn about a vertical axis into a retracted position wherein the claw member lies constrained under the flange to permit insertion of the sink unit into the countertop opening. The guide member ensures that as the claw member rises up the bolt the claw member turns about the vertical axis into a constrained outwardly extended position for engagement with the underside of the countertop.