Siyahoo district in Hormzgan province is known as one of the most important citrus producing areas of Iran. Siyahoo mandarin is a popular variety of mandarin (Citrus reticulata) cultivated in this region. During frequent visits in 2017 and 2018, following observed symptoms of Hop Stunt Viroid (HSVD) including severe inverse stem pitting symptoms in trunk and crown tissues of some mandarin trees, we studied existence of the causal agent of the drastic decline of mandarin trees in citrus orchards of Siyahoo. To confirm cachexia viroid (HSVD) infection, leaf and crown samples were collected from five severely symptomatic mandarin trees and the total RNA was extracted using RNX-Plus solution (SINACLON, Iran). The isolated RNA was subjected to reverse transcription (RT) PCR assay with specific primers (HSVDf and HSVDr) to detection cachexia viroid. The HSVD was detected in three leaves and five crown samples of the symptomatic mandarin trees. But, plant samples from symptomless trees was not positive for viroid infection. One of the positive PCR product was directly sequenced and edited. BLASTn search of the sequence showed 97 % identity with sequences of Hop Stunt Viroid (accession no: AF213487, AF131249 and KX156935) obtained from NCBI. HSVD is known as causal agent of the cachexia, an economically important disease of susceptible citrus species such as tangelo and mandarin. Symptoms of the disease is including discoloration, gumming and browning of phloem tissue, wood pitting and bark cracking. To our knowledge, this is the first report of HSVD in mandarin trees in Hormozgan province.