Abstract: An adapter designed to sealably couple a standard baby bottle nipple and cap to a beverage bottle, such as but not limited to a plastic water bottle. The adapter includes a generally cylindrical shaped base having a passage therethrough facilitating fluid flow. The adapter further includes a sealing disc extending in a perpendicular plane from the base and is of sufficient diameter to engage with the threads of the cap. The sealing disc engages with at least a portion of the bottom of the nipple disposed within the cap to substantially inhibit leakage. The adapter further includes a lip circumferentially disposed around one end of the base that is proximate the nipple when the sealing disc is engaged with the cap.