Trichodorus arasbaranensis n. sp. is described from the rhizosphere of Carpinus betulus growing in forests in Arasbaran, north-west Iran. The new species is characterised by its large body length (1020-1520 μm in males; 1100-1500 μm in females), cuticle abnormally swollen upon fixation, onchiostyle medium-sized (41-66 μm in males; 49-65 μm in females) and secretory-excretory pore at level of pharyngeal bulb. Males have a single ventromedian cervical papilla varying in position from anterior to posterior to secretory-excretory pore, sperm cells with sausage-shaped nucleus, large (74-110 μm), smooth, ventrally curved spicules, manubrium wide, not offset from lamina, lamina narrowed in mid-region and with a distal ventral velum, and gubernaculum 25.5-34.5 μm long. Females have a short, but well developed, rounded vagina with large, rounded, triangular to oval-shaped, sclerotised vaginal pieces in lateral view. Because of its large body size, the new species most closely resembles T. magnus, T. elegans, T. obtusus, T. californicus and T. nanjingensis, from which the female can be differentiated by the short vagina length in relation to the corresponding body diam. and large size of the vaginal sclerotised pieces, and the male by the longer and differently shaped spicule which bears a velum. A key to the males and females of the Trichodorus spp. recorded from Iran is provided.