Based on literature survey, specimen examination and field work, together with the observations of achene micromorphology, perigynium micromorphology and leaf anatomy, this monograph presents the delimitation and revision of Carex sect. Rhomboidales (Cyperaceae). Forty species with six subspecies and four varieties are recognized in this monograph, including a new species Carex pengii X. F. Jin & C. Z. Zheng. Four subspecies and one variety are combined: Carex harlandii subsp. khoi (T.V. Egorova & Aver.) X.F. Jin, C. canina subsp. collifera (ohwi) X.F. Jin, C. wahuensis subsp. sakonis (T. Koyama) X.F. Jin, C. longirostrata subsp. tsinlingensis (K.T. Fu) X.F. Jin, and C. rhynchophora var. chorda (H. Lev. & Vaniot) X.F. Jin. Lectotypes are designated for C. basiflora C.B. Clarke, C. brevicuspi C. B. Clarke, C.boottiana Hook. & Arn., C. harcei C.B. Clarke, C. harlandii Boott, C. simulans C.B. Clarke, C. wilfordii C.B. Clarke and C. wahuensis C.A. Mey. subsp. robusta (Franch. & Sav.) T. Koyama. Twenty-eight names of taxa are reduced as synonyms. All recognized species, subspecies and varieties are described with illustrations and specimen cited.