A water supply device for a planting container comprises a main body having a water storage space and a water supply space. A water supply hole is disposed between the water storage space and the water supply space. Water in the water storage space is transported to the water supply space through the water supply hole. A water absorbing material is disposed in the water supply space and utilizes siphon principle to pump the water in the water supply space. When the water level of the water supply space is higher than the water supply hole, a vacuum effect is generated. When the water level of the water supply space is lower than the water supply hole, the vacuum effect is released so that water in the water storage space is transported to the water supply space through the water supply hole.
