Phytophthora crown and root rot annually kills numerous productive and non-productive pistachio trees. Once the pathogen is established in an orchard, it can lead to the progressive death of trees over several years, rendering the orchard economically unviable. One of the important strategies for biological control of Phytophthora utilizes Trichoderma fungi, which are commercially applied in many regions worldwide. To implement the study and isolate and identify Trichoderma fungi, soil samples were collected from the rhizosphere of productive trees (15–25 years old), pistachio seedlings, and weeds in key pistachio-growing areas of Semnan Province between 2008 and 2014. Sampling was performed randomly at depths of 5–30 cm. The collected samples were then mixed to create a composite sample and transported to the laboratory. After preparing the soil suspension, 1 mL from each of the 10⁻¹ and 10⁻² dilutions was spread onto both acidified PDA and TSMC media in four replicates. The Petri dishes were incubated in darkness at 25°C for one week, and Trichoderma colony populations were examined daily. The results showed that Trichoderma fungi were isolated from 30% of the soil samples. Also, micromorphological characterization of the isolates indicated that the obtained Trichoderma isolates belonged to five species: Trichoderma harzianum, Trichoderma longibrachiatum, Trichoderma atroviride, and Trichoderma brevicompactum. Among these isolates, T. harzianum had the highest abundance. The aim of this study was to isolate and identify Trichoderma species as biocontrol agents against Phytophthora fungi from the soil of pistachio orchards in Semnan Province.