OCTOPUS is an open-source, motion-activated underwater camera system designed for long-duration monitoring of marine organisms in their natural habitat. The system can operate continuously for approximately 72 hours and has been deployed at depths of up to 800 feet, allowing researchers to observe behavior that may be difficult to capture during short diver surveys. OCTOPUS has been used to document cryptic and den-associated species, including Octopus rubescens, and to record patterns of activity, den use, species interactions, and other ecological behaviors with minimal disturbance from observers.