To survive in the host cells, viruses have to adapt various strategies, which include the modulation of microRNA (miRNA) pathway through virus-encoded miRNAs to modulate the host cellular environment. It has been shown that several avian viruses, mostly herpesviruses, encode a number of miRNAs. These include 26 miRNAs encoded by the highly oncogenic Marek’s disease virus-1, 36 miRNAs encoded by avirulent Marek’s disease virus-2, 28 miRNAs by herpesvirus of turkeys, 10 miRNAs by infectious laryngotracheitis virus, 41 miRNAs by duck enteritis virus, and 2 miRNAs by avian leukosis virus subgroup J. Although locations of some of the miRNAs are conserved within the repeat regions of the genomes among some of the antigenic and phylogenetic closely related herpesviruses, there are no sequence conservation of miRNAs encoded by different avian herpesviruses. Moreover, some of the virus-encoded miRNAs have the same seed sequence as host miRNAs serve as functional orthologs. For example, mdv1-miR-M4-5p, a functional ortholog of gga-miR-155, is critical for the Marek’s disease virus in inducing tumors. In this review, we describe the advances in our understanding on the role of the herpesvirus-encoded miRNAs in avian diseases. Additionally, we also describe the potential association of avian leukosis virus subgroup J encoded E (XSR) miRNA in the induction of myeloid tumors in certain genetically distinct chicken lines.