Tylenchulus semipenetrans Cobb, 1913 is among some of the most economically important plant-parasitic nematodes in the world. The nematode has been identified as the causal agent of slow decline. Most studies estimate the yield losses, due to T. semipenetrans, to range from 10% to 30%, depending on the level of infection, aggressiveness of the nematode population, soil characteristics, susceptibility of the rootstock, presence of other pathogens and grove management practices. In order to identify the citrus nematode in northern Iran, soil and root samples were collected from infected trees. The second-stage juveniles were isolated from the soils by the tray method. Eggs and females were extracted from roots by the centrifugal-flotation technique. Morphological observations and molecular evidence confirmed this population as T. semipenetrans. A phylogenetic tree of T. semipenetrans populations was reconstructed based on 28S rRNA gene sequences using RAxML. Morphologically, there is a slight difference between the studied population and the reported populations of T. semipenetrans from pomegranate and banana orchards in southern Iran. Phylogenetic analysis showed the close relationship of the T. semipenetrans population from northern Iran with other populations of this species. Based on molecular analysis, Tylenchulus was identified as a monophyletic group. The phylogenetic position and measurements of T. semipenetrans were provided.