Seed morphology provides some of the most important characters for species delimitation in Justicia (Acanthaceae). The seed morphology of 30 species from Thailand was studied using stereomicroscopy and scanning electron microscopy. Variation was seen in size, color, and shape. The range in seed size is 0.71–4.52 mm in length and 0.74–4.48 mm in width. The color varies from pale to dark brown, pale yellow, yellowish red, or dark green. The shape varies from ovoid to ellipsoidal, orbicular, suborbicular, or heart-shaped. Five different morphological types are proposed according to features of the testa. The seed morphology of each type is described, illustrated, and discussed according to its taxonomic significance. A key to identify the different types is also presented.