The present regional monograph is based on the study of 4,859 specimens of Cerithiopsidae and Newtoniellidae collected during several expeditions in New Caledonia and the Loyalty Islands. Altogether, 171 species are present of which 76 are new to science; the range of 92 already known species is expanded; 3 species are figured but not named. The generic allocation of a number of species remains uncertain, a situation that can be resolved only with the analysis of the soft parts. All species are described and illustrated with digital color photos and with environmental scanning electron microscope (ESEM); particular emphasis was given to protoconchs. Fifteen genera of Cerithiopsidae are represented: Belonimorphis (1 species), Cerithiopsidella (1), Clathropsis (22), Horologica (22), Joculator (41), Marshallopsis (12), Medax (5), Onchodia (1), Prolixodens (10), Retilaskeya (1), Seila (3), Specula (16), Sundaya (7), Synthopsis (18), Tubercliopsis (3); and 2 genera of Newtoniellidae: Ataxocerithium (2), Cerithiella (6). We also analyzed the species from Lifou and Uvea (Loyalty Is.) instituted by Melvill & Standen (1896).