The succulent leaves of Hallianthus planus are paired, only slightly fused at the base and triangular in cross-section. They are strongly keeled and sometimes laterally compressed. The leaf keels are rounded at their tips.

Initially short and stubby, old leaf-pairs become slightly sickle-shaped and elongated, up to 3 cm long. Faintly shiny surface cells cover the otherwise smooth or mottled leaf skins, when not wrinkled from moisture loss. Young leaves may have a paler, green to pinkish grey colour, the mottling subtler (Williamson, 2010; Leistner, (Ed.), 2000; Smith, et al, 1998; iNaturalist).