Scyphozoans are carniverous.  Some are active predators, and others are filter-feeders.  They mostly eat plankton and the larvae of fish and other organisms.  Predatory scyphozoans catch prey using their tentacles, which are armed with touch-triggered cnidoblasts that release venemous nematocysts.  The nematocysts pierce the prey and immobilize it, and the jellyfish then uses its oral tentacles to bring the food into their mouth.  The filter-feeders catch and eat food particles that are suspended in the water.  Scyphozoans do not have an anus, so after the food is digested in the ciliated digestive cavity, waste is released through the same opening the food was introduced through.

The order Rhizostomeae differ in their feeding morphology from the other Scyphozoans.  Instead of a large central mouth, this order has many smaller mouths on the oral arms.  One member of this order, Casseopia, is unique in that it has symbiotic algal dinoflaggelates in its body tissue, and derives most of its energy from the carbohydrates that the algae fixes.  Casseopia is a tropical species, and relies on sunlight so that the algae can photosythesize.

Scyphozoans can be very aggressive predators.  They can cause populations of other organisms to decline either by competing with them for resources or by preying on their young.  Because they play such a large role in ecosystem dynamics, scyphozoan population size has been cited as a parameter for determining habitat health.