This data publication contains sea-surface temperatures and diatom abundances and biometry in sediment core DCR-1PC from the southwestern Indian sector of the Southern Ocean over the last 350,000 years. Core DRC1-PC was retrieved during the oceanographic expedition Hakuho-maru KH-10-7 during the austral summer 2010-2011. Diatom census counts and diatom biometry were performed in 2018-2019 to investigate the potential impact of climate degradation on Hemisdicus karstenii disappearance at ~190 ka.Included are relative abundances (RA in % of the total diatom assemblages) and absolute abundances (AA in millions of valve per gram of dry sediment) of Hemidiscus karstenii (Hk) and Fragilariopsis kerguelensis (Fk). Summer sea-surface temperatures (January to March) were estimated by applying the Modern Analog Technique (MAT) to fossil diatom assemblages. Hk and Fk biometry versus depth and age are also included. For each depth, mean length results from measurements of 100 different and complete valves of both Hk and Fk on which within-sample standard deviations were calculated.