The spionid polychaete Polydora rickettsi is primarily a borer in various calcareous substrata in the eastern Pacific. Redescription of the species from the type locality in Mexico is required to better understand its distribution. In Chile, females and males become mature after growth to c. 60 segments. Sex allocation in the population is close to 1:1. Gametes develop along segmental blood vessels in middle segments. Spermatogonia proliferate in temporary testes and the rest of spermatogenesis occurs in the coelomic cavity. On their release through metanephridial segmental organs, longheaded spermatozoa are packed in filiform spermatophores. Females store inactive spermatozoa in paired seminal receptacles on the dorsal side of fertile segments. Oogenesis is mostly intraovarian. Females deposit up to 4150 eggs into 65 capsules which are joined to each other in a string. Each egg capsule is attached by two stalks to the inner wall of the burrow and contains up to 65 eggs c. 95 μm in diameter. Most eggs give rise to larvae which develop inside egg capsules until the 3‐segment stage, then hatch and continue development in sea water, feeding on plankton. Fully developed 17–18‐seg‐ment larvae are able to settle and metamorphose. They have one pair of phaosomes and three pairs of black eyes on the prostomium, provisional proto‐nephridia in segments 1 and 2, paired dorsal melanophores from segment 3 onwards, lateral melanophores on segment 2 and from segment 10 onwards, yellow pigment in the wall of posterior gut, glandular pouches from segment 6 onwards, short branchiae on segments 7–11, gastrotrochs on segments 3, 5, 7, 10, 13, 15, and 17, one pair of provisional modified bristles and two or three falcate spines of a “quasi‐adult” kind in segment 5. Development of the “quasi‐adult” spines is described for the first time in polydorid larvae.