Fungi are frequently infected with viruses called mycoviruses. Some mycoviruses have shown potential as biocontrol agents because they can weaken the virulence of fungal plant pathogens. Intensive research has been conducted on the diversity of viruses infecting plant pathogens and the functions of such viruses. In contrast, the diversity of viruses infecting fungi that are not plant pathogens is less well understood. We previously identified a mycovirus of the family Partitiviridae, Aspergillus sulphureus partitivirus 1 (AsuPV1), from an Aspergillus sulphureus strain isolated from fermented dried bonito. In the present study, we conducted a detailed phylogenetic analysis of AsuPV1 and determined its molecular characteristics. The phylogenetic analysis revealed that AsuPV1 belongs to the proposed subgroup I within the genus Gammapartitivirus. Virus particles of AsuPV1 were isolated by density gradient centrifugation, and molecular analyses indicated that each of its three genome segments is encapsulated in capsids composed of the putative viral coat protein. AsuPV1 is the second virus with a tri-segmented genome to be found in a member of Gammapartitivirus subgroup I, after Penicillium stoloniferum virus F. A phenotypic comparison between the parental A. sulphureus strain and isogenic virus-free isolates indicated that AsuPV1 enhances conidium production in its host. This study provides insight into the diversity and molecular characteristics of partitiviruses with a tri-segmented genome.