J. smithi has a slightly compressed, oblong body with two dorsal fins, the first having a deeply notched membrane with the third and fourth spines being the longest.

It is light tan in colour, becoming paler towards the whitish belly, with five or six faint, dark bars which fade completely in larger specimens.

[2][3] J. smithi has an Indo-Pacific distribution from the Red Sea to the Marshall Islands, north to Taiwan and south to Indonesia.

[4] Smith's cardinalfish is a nocturnal species that hides among rocks and reefs during the day and emerges at night to feed on invertebrates over sandy to muddy substrate at depths of 30–50 m. It spawns during the warmer months and is a mouthbrooding species, which shows pairing behavior during courtship and spawning; the males are responsible for incubating the eggs.

[3] The specific name honours the South African ichthyologist James Leonard Brierley Smith (1897-1968), who in 1961 wrote a monograph on the cardinalfishes of the Indian Ocean in which he proposed the genus Jaydia.