Sesbania tomentosa is an erect, prostrate shrub with decumbent or sprawling branches that can reach up to 14 meters long. Sometimes, it may form small, erect trees standing 2.5–6 meters tall. It features dense, silky villous covering, featuring somewhat spreading hairs on new growth and often appressed hairs on older stems. The leaves are compound, even-pinnate, and alternate, with (12–)18–38 oblong-elliptic to narrowly elliptic leaflets, each 15–38 mm long and 5–18 mm wide. The leaflets’ apex can be rounded, subtruncate, or weakly emarginate and apiculate, while the base is typically rounded to broadly cuneate, often oblique. Surfaces of the leaflets range from densely to sparsely silky strigose, sometimes becoming glabrate, especially on the upper surface, or even glabrous, with entire margins. The flowers are papilionaceous, appearing 2–9 in axillary racemes measuring 1.2–7(–11) cm in length. The pedicels are articulated at the apex, ranging from 9–40 mm long, with a rachis and pedicels densely covered in silky villous hairs. The calyx is campanulate, 5-toothed, and 7–12(–15) mm long, varying from strigose to glabrous, with deltate lobes and 1.5–4 mm long. The corolla is distinctively salmon-tinged yellow, orangish red, or scarlet, with the standard petal measuring 23–45 mm long. The plant bears linear pods 7–230 mm long and 4–6 mm wide, slightly flattened and often somewhat constricted between the seeds. The fruits are linear pods; 7–230 mm long; 4–6 mm wide; slightly flattened; often somewhat constricted between the seeds; indehiscent or very tardily dehiscent; transversely septate and slightly constricted between the seeds; apex beaked. The Seeds are (4–)6–27 per fruit; olive to pale or dark brown; oblong–ellipsoid to oblong; 4–6 mm long; 2–3 mm wide. Formerly widespread in dry habitats, it is now limited to relictual populations on sandy beaches, dunes, soil pockets on lava, and along pond margins due to habitat destruction at elevations ranging from sea level up to 830 meters.