Twenty-one species of Chaetomium known from Iran were compared on the basis of morphological and molecular characters. Six new species are recognized, five isolated from cereals and one from nematode cysts. A combined sequence dataset of the ITS region, partial LSU rDNA, and β-tubulin gene sufficiently resolved five species groups of Chaetomium that are largely concordant with combined features of peridium structure, ascospore shape and germ pore position. Among the new species C. undulatulum is a close relative of C. globosum, C. rectangulare is close to C. elatum, C. interruptum and C. grande are close to C. megalocarpum, altogether forming the C. globosum species group. Chaetomium iranianum and C. truncatulum are members of the C. carinthiacum species group, characterized by spirally coiled ascomatal hairs and fusiform ascospores. A chrysosporium-like anamorph is newly described for C. acropullum.