The adult males grow up to 28–36 millimetres (1.1–1.4 in) long, while females reach 32–42 millimetres (1.3–1.7 in). This insect is most often completely green (but there are specimens completely yellowish or with yellow legs), excluding a rust-colored band on top of the body. The organ of the stridulation of the males is generally brown.
Tettigonia viridissima is distinguished by its very long and thin antennae, which can sometimes reach up to three times the length
of the body, thus differentiating them from grasshoppers, which always carry short antennae. It could be confused with Tettigonia cantans, whose wings are a centimeter shorter than the ovipositor, or Tettigonia caudata, whose hind femurs bear conspicuous black spines.
The morphology of both sexes is very similar, but the female has an egg-laying organ (ovipositor) that can reach a length of 23–32 millimetres (0.91–1.26 in). It reaches the end of the elytra and is slightly curved
downward.
The larvae are green and the imago show a thin brown longitudinal stripe on their back. The ovipositor can be seen from the fifth stage; the wings appear in both genders from the sixth stage.