The genetic diversity of 77 Trichoderma harzianum isolates collected from sunflower rhizosphere soils in Urmia, Khoy, and Salmas in West Azerbaijan province, Iran, was evaluated by using the Universal Rice Primer (URP) molecular marker. The DNA band pattern of the isolates was developed using seven primers of this marker. These primers produced 186 gene loci, out of which 182 loci were polymorphic. Accordingly, the genetic diversity of the isolates was calculated, and their kinship relations were determined by cluster analysis using the NTSYS software package. URP-6R had the highest marker index among the studied primers, followed by URP-1F, URP-4R, and URP-25F, implying their higher efficiency in discriminating between the isolates. The results showed that the URP marker could discriminate between isolates using macroscopic morphological characteristics, such as color and colony type, potential of pigment production in the culture medium, and colony growth rate. Furthermore, there was no significant relationship between the geographical distribution of the isolates and the band patterns generated by the primers except for a few cases. The results generally revealed that the URP marker was an efficient tool for determining the genetic diversity of T. harzianum.