Sporolithon ptychoides is a species of crustose red seaweed with a hard, calcareous skeleton in the family Corallinaceae.

In the southwestern Atlantic off the coast of Brazil, Sporolithon ptychoides forms rhodoliths up to 15 cm (6 in) in diameter.

In the southwestern Indian Ocean off the coast of Natal, this species does not form rhodoliths, instead being crustose, with flat or knobbly thallus sometimes loosely and sometimes securely attached to the substrate.

This is one of several species of crustose coralline red algae that form rhodoliths on the seabed in the southwestern Atlantic.

Rhodoliths are clumps of calcareous material that resemble corals but are not attached to the substrate.