Three bacterial strains, W99^T, S113^T, and S268^T, were isolated from marine algae and subjected to polyphasic taxonomic characterization. Phylogenetic and phylogenomic analyses placed W99^T within the genus Shimia, whereas S113^T and S268^T formed two distinct genus-level lineages within the family Roseobacteraceae. The three isolates were Gram-stain-negative, non-motile, and obligately aerobic, with optimum growth at 25, 25, and 30°C, respectively, and at 2.0% (w/v) NaCl. Ubiquinone-10 was the sole respiratory quinone in all three strains. The major fatty acids were 11-methyl C_18:1 ω7c and summed feature 8 (C_18:1 ω7c and/or C_18:1 ω6c) in strain W99^T; C_16:0 and summed feature 8 in strain S113^T; and 11-methyl C_18:1 ω7c, cyclo-C_19:0 ω8c, and summed feature 8 in strain S268^T. The major polar lipids were phosphatidylglycerol and phosphatidylcholine, with diphosphatidylglycerol additionally detected in strain S268^T. Polyphasic taxonomic analyses demonstrated that strain W99^T represents a novel species of the genus Shimia, for which the name Shimia rhodophyticola sp. nov. is proposed (W99^T = KACC 24154^T = JCM 38196^T), whereas strains S113^T and S268^T represent two novel genera and species, Neomarinovum algicola gen. nov., sp. nov. (S113^T = KACC 23417^T = JCM 38210^T) and Phycorhabdus gracilariae gen. nov., sp. nov. (S268^T = KACC 24156^T = JCM 38211^T), respectively. In addition, comprehensive phylogenomic and genome-based relatedness analyses support the taxonomic revision of nineteen taxa within the family Roseobacteraceae, providing a revised genome-based taxonomic framework that better reflects the evolutionary relationships within the family.