endothermic reactions take in energy and end with a higher energy content than they started with.
The photosynthesis reaction requires both energy from the sun and the use of ATP to move the reaction forward. However the result of photosynthesis is more stored energy than the process requires.
The reverse reaction of photosynthesis is cellular respiration. The result of cellular respiration is heat and 36 ATP molecules. So cellular respiration is exothermic.
That photosynthesis reactions taking in energy and the fact that the reverse reactions is exothermic show that the photosynthesis is an endothermic reaction.