A novel actinobacterial strain, designated as JXJ CY 30 ^T, was isolated from the phycosphere of Microcystis aeruginosa FACHB-905 (Maf) collected from Lake Dianchi, China. The strain was a Gram-stain-positive, aerobic and coccus-shaped actinobacterium. It had alanine, glutamic acid, aspartic acid, and lysine in the peptidoglycan, and mannose, ribose and arabinose in its cell wall sugars, anteiso-C_15:0 and iso-C_15:0 as the main cellular fatty acids, MK-7 and MK-8 as the major respiratory quinones, and phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol, glycolipid, and an unidentified phospholipid as the polar lipids. The DNA G + C content was 73.08%. Its 16 S rRNA gene sequence shared 99.14%, and 98.75% similarities with Micrococcus flavus DSM 19079 ^T and M. porci KD337-16^T, respectively, and ≤98.41% similarities with other type strains of the genus Micrococcus. It formed independent clade with M. flavus DSM 19079 ^T on the phylogenetic trees. The digital DNA-DNA hybridization and average nucleotide identity values between strain JXJ CY 30 ^T and M. flavus DSM 19079 ^T and M. porci KD337-16^T were 48.0% and 92.1%, 25.5% and 83.2%, respectively. These data above indicated that strain JXJ CY 30 ^T represented a new species of the genus Micrococcus, and the species epithet is proposed as Micrococcus lacusdianchii sp. nov. (type strain JXJ CY 30 ^T = KCTC 49378 ^T = CGMCC 1.17508 ^T). Strain JXJ CY 30 ^T can potentially provide Maf with various nutrients such as available phosphorus and nitrogen, plant hormones, various vitamins and carotenoids for growth, while it was inhibited by metabolites from its symbiotic algae Maf.