Portulaca molokiniensis is a stout perennial herb. The older stems have a pale, corky secondary growth layer and are usually branched at or slightly beneath the ground, forming clumps 25–50 cm in diameter. The leaves are simple and can be alternate or opposite, clustered along the apical portion of the stem. The blades are imbricate, decussate, and broadly obovate, measuring 40–45 mm long and 20–55 mm wide. The upper leaves occasionally form an involucre around the flowers. The leaves are succulent, with a 1.5–3 mm long tuft of hairs in the axil. The leaf margins are entire, and the leaves are sessile or subsessile. There are two stipules, which are connate basally. The flowers are in dense headlike clusters at the ends of leafless internodes 5–13.5 cm long, rarely with 1–2 leaves. Four involucrate leaves closely subtend the inflorescence. The flowers are bisexual (perfect), actinomorphic (radially symmetric), rarely slightly irregular, and sessile. The calyx consists of 2 sepals: imbricate, persistent, and 8–10 mm long with a rounded apex. The corolla has 4–6(–8) petals that are bright lemon yellow and obcordate, measuring 12–18 mm long, distinct or connate at the base, and imbricate. There are 40–55 stamens in clusters inserted on the nectary disk, with a dense mat of yellow papillae at the base. The filaments are usually puberulent, and the anthers are dithecal, opening by longitudinal slits. The fruit is a membranous capsule, ovoid, 8–9 mm long, and circumscissile slightly below the middle. The seeds are dark brown and reniform, 0.9–1.1 mm long, with cells of the seed coat having blunt-tipped spines that are 0.05 mm long. The surface of fresh seeds is glistening and somewhat sticky. It is found in volcanic tuff, detritus at the base of sea cliffs, and on steep rocky slopes between 10-115 meters.