A retainer for a bottle cap. In one implementation, the retainer includes a first connector at a first end, the first connector being made of a first elastic material. The first connector defines a first opening having an inner perimeter sized to be captured in a groove in a particular bottle in a slip fit. The retainer also includes a second connector at a second end, the second connector being made of a second elastic material. The second connector defines a second opening having an inner perimeter sized to receive an outer perimeter of a cap of the particular bottle in an interference fit. The retainer further includes a flexible elongate strap joining the first and second connectors.