Even though CO2 enrichment is known to effectively promote photosynthesis and is widely used around the world, the use of this technique is uncommon in Japan. The aim of this study was to develop a new air-conditioning system for greenhouses by using a high-efficiency, low-cost cooling technique that allows CO2 enrichment during the high-temperature phase. We developed an air-conditioning system using CO2 clathrate hydrate and evaluated its performance in the laboratory. The capacity of the hydrate was sufficient to stably increase the CO2 concentration, and photosynthesis was promoted in experimental tomato plants for a given length of time. The amount of cold energy supply from the hydrate was not sufficient to completely replace electrical cooling for the entire experimental period, but this deficiency most likely can be overcome by combining this type of cooling and CO2 enrichment system with other innovative methods of cooling.