Species of Chaetoceros sect. Stenocincta are frequently observed in marine phytoplankton. To study species diversity within the section Stenocincta, monoclonal strains were established from Chinese coastal waters and Norwegian Sea. Following detailed morphological observations and molecular evidence inferred from nucleus-encoded large subunit ribosomal DNA, the descriptions of C. affinis and C. willei are emended based on isolates from their respective type localities. Additionally, three new species are described. Chaetoceros donghaiensis sp. nov. is characterized by short straight chains, with thin setae only, and terminal setae that tend to cross distally. Chaetoceros forceps sp. nov. is unique by its thick terminal setae, each with an inflation near the end, with each pair forming a V-shape, resembling forceps. Chaetoceros lepidus sp. nov. is characterized by short, straight chains, narrowly elliptical valve faces, setae all located in the apical plane, and arched intercalary setae in opposite directions to the chain axis in broad girdle views. Three other distinct clades, which may also represent new species, were discerned in the LSU rDNA phylogeny, but more morphological characters are needed before formal proposals can be made. These species/clades formed a well-supported monophyletic lineage, supporting the current circumscription of section Stenocincta. This section is characterized by colonies in straight chains, cells with one chloroplast only, narrow lanceolate apertures, and terminal setae generally thicker than intercalary ones or both similar and thin. However, in C. diversus the terminal setae are thin and distinctly thicker pairs of intercalary setae are usual.