Jacquemontia sandwicensis is a vine with prostrate, herbaceous stems that often root at the nodes. These stems can grow up to 3 meters long and range from subglabrous to densely tomentose, sometimes becoming glabrate with overlapping 2-branched hairs. The leaves are simple and alternate, with elliptic to suborbicular blades, 4–6 cm long and 1.5–3.5 cm wide. The leaves are thick and fleshy, with a retuse apex and a cuneate to attenuate base. The surfaces can be glabrous or pubescent with stellate hairs, and the margins are entire. Petioles measure 5–15 mm long, and stipules are absent. The flowers are few in axillary cymes, rarely solitary, with 5–10 mm long pedicels and linear or obovate bracts 3–8 mm long. The flowers are bisexual (perfect), actinomorphic (radially symmetric), and 5-merous. The calyx has five unequal sepals: the outer two are broadly ovate, broadly obovate, or broadly elliptic, 5–8 mm long, 4–6 mm wide; the middle sepal is ovate or falcate, 5–6 mm long, 2–5 mm wide; and the inner two are narrowly ovate, 4–6 mm long, 2–3 mm wide. Sepals are distinct, imbricate, pubescent, or glabrous, with a persistent, acute to acuminate apex and, occasionally, accrescent. The corolla has five fused petals: pale blue or white, broadly campanulate, 7–15 mm long, deeply lobed, dentate or subentire, and either glabrous or pubescent. The nectary disk is small or absent. Stamens are distinct with 5–12 mm long filaments, inserted on the corolla tube base, and alternate with the corolla lobes. Stamens and styles are included, with dithecal anthers, usually linear or oblong, 1–2 mm long, and extrorse. The ovary is superior and 2-celled, with four ovules, placentation that is basal or basal-axile, anatropous, and either glabrous or pubescent. The style is filiform, simple, or bifid, 7–13 mm long, with stigma lobes 1–1.25 mm long. The fruit is a capsule that is tan to brown, 2-celled, 4-valved, or 8-valved, quadrangular-globose, 4–6 mm in diameter, and enclosed by somewhat enlarged sepals. It dehisces by valves transversely, irregularly, or can be indehiscent, berry-like, or nut-like. The seeds are 1–4 in number, trigonous, 2–3 mm long, with or without winged margins, and have a seed coat that is minutely areolate and sometimes faintly ruminate. The endosperm is absent or scanty. It occurs in coastal habitats on various substrates, particularly on the leeward sides of islands between 0–30(–550) meters.