Abstract: Apparatus for securing a cap of a container to an external structure includes retaining structure configured to fit inside and engage the cap when the apparatus is installed. Securing structure connected to the retaining structure is used to releasably secure the cap to the external structure. In one embodiment, the retaining structure is a coil sized to exert a spring tension against the interior of the cap. In another embodiment, the retaining structure is an eyelet sized to receive a fastener secured within the cap. The securing structure may be comprised of a hook. Alternatively, the securing structure may be comprised of a clip or other closure mechanism. The apparatus may further include an intermediate bend providing a space that accommodates a feature of the cap, such as a vent in a fuel tank cap. The apparatus may remain in the cap when the cap is replaced on the container.