The main waste product of photosynthesis is oxygen gas (O₂). When green plants, algae, and some bacteria carry out photosynthesis, they use carbon dioxide and water to make glucose (a sugar) using light energy, and in the process they release oxygen as a byproduct. This oxygen is “waste” for the plant itself (it doesn’t need that extra O₂ from this reaction), so it is released into the air—where it becomes essential for animals and many other organisms that rely on it for respiration. In simple word-equation form:

carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

So if you’re answering a textbook or quiz question like “what is photosynthesis waste product?” , the expected answer is: oxygen (O₂).