The lemon-yellow tree frog (Hyla savignyi) is a medium-sized frog with long hind limbs and slight webbing between the digits on the feet. The tips of the digits are expanded into discs, which are smaller on the hind limbs than on the forelimbs. The back of the lemon-yellow tree frog is smooth, while the throat and underside have a rough texture.

As its common name suggests, the lemon-yellow tree frog is typically yellowish to light green, but variations of brown and straw colouration can also exist. Its overall colouration varies depending on factors such as the external temperature and the colour of the substrate that the frog lives on. The underside of the lemon-yellow tree frog is white, and a dark line extends along the side of the body from the nostrils to the hind legs.

The male and female lemon-yellow tree frog are very similar in appearance, but can be distinguished from one another by the large vocal sac that is visible externally on the male frog, appearing as darker folds and wrinkles of skin on the throat.

The lemon-yellow tree frog was previously considered to be a subspecies of the common tree frog (Hyla arborea), but differs slightly in its colouration and in its calls.