Endophytic fungi are characterized as microorganisms found within internal tissues of living plants without any immediate, overtly negative effects. The present study was carried out to isolate, taxonomically characterize and determine the spatiotemporal distribution of endophytic fungi associated with leaf, stem, trunk, and root of mandarin (Citrus reticulata cv. Siyahoo). To do so, the sampling program was done seasonally in four geographically isolated mandarin growing areas of Hormozgan province of Iran, including Siyahoo, Ahmadi, Sikhoran, and Roudan. In total, 702 fungal isolates were obtained from leaf, stem, trunk, and root of healthy mandarin trees divided into 26 distinct morphotypes based on morphological characteristics. The morphotypes were taxonomically characterized through phylogenetic analysis of the ITS1-5.8S-ITS4 rDNA region sequences. Accordingly, 10 different fungal orders from 5 fungal classes were identified, i.e., Saccharomycetes (Saccharomycetales), Eurotiomycetes (Eurotiales), Dothideomycetes (Capnodiales, Pleosporales, Dothideales), and Sordariomycetes (Diaporthales, Hypocreales, Microascales, Togniniales), all from Ascomycota, which represented 97.2% and Ustilaginomycetes (Ustilaginales) from Basidiomycota which represented 2.8% of the isolates. The Aureobasidium pullulans, Penicillium citrinum, and Dothideomycetes sp. were the most frequent isolates. The trunk and leaf showed the highest and lowest total colonization frequency and species richness of endophytic fungi, respectively, in all sampling periods. The results showed that the colonization frequency of endophytes in Hormozgan province was higher in autumn than that in spring, winter, and summer. The trunk showed the maximum diversity of endophytes over all seasons. The Shannon-Wiener (H') and Simpson indices had significant correlation with sampling cites and tissue type and the maximum value of Shannon and Simpson indices (H' = 3.05 and 1 - D = 0.94) was found in the specimens collected from Siyahoo. In conclusion, the three factors (season, location, and tissue type) all in together could determine fungal endophyte composition of C. reticulata.