A paint cup for a spray gun includes a container containing paint solvents, a lid body covering an upper opening of the container, and a ventilation member mounted in the container. The lid body includes an annular coupling portion protruding from an inner surface thereof, an inner rod located in the annular coupling portion, and a vent extending into an annular coupling groove of the annular coupling portion. A gap is formed between the ventilation member and the inner rod. When the container is turned upside-down, the paint solvents in the gap will flow into an accommodating groove formed between the inner rod and the annular coupling portion, and ambient air circulates in the container via the vent and the gap.