Strain 1 T , isolated in the seventies from the thallus of the carrageenophytic red algae Eucheuma spinosum collected in Hawaii, USA, was retrospectively characterized using phenotypic, phylogenetic and genomic methods. Bacterial cells were Gram-stain-negative, strictly aerobic, non-flagellated, coccoid, ovoid or rod-shaped, and grew at 10-42 °C (optimum 20-25 °C), at pH 5.5-10 (optimum pH 6-9) and with 2-12 % NaCl (optimum 2-4 %). Strain 1 T grew on the seaweed polysaccharides i-carrageenan, laminarin and alginic acid as sole carbon sources. The major fatty acids (>10 %) were C16:0, C18:1 ω7c and summed feature 3 (C16:1w7c and/or iso-C15:0 2OH) and significant amounts of C16:0 N alcohol (6.7 %) and 10 methyl C17:0 (8.6 %) were also present. The only respiratory quinone was Q-8, and major polar lipids were phosphatidylethanolamine, phosphatidylglycerol and an unknown aminolipid. Phylogenetic analyses based on 16S rRNA gene sequence comparisons showed that the bacterium is affiliated to the genus Alteromonas (family Alteromonadaceae, class Gammaproteobacteria). Strain 1 T exhibits 16S rRNA gene sequence similarity values of 98.8-99.2 % to the type strains of Alteromonas mediterranea and Alteromonas australica respectively, and of 95.7-98.6 % to those of the other species of the genus Alteromonas. The DNA G+C content of strain 1 T is 43.9 mol%. Based on the genome sequence of strain 1 T , DNA-DNA hybridization predictions by the average nucleotide identity (ANI) and Genome-to-Genome Distance Calculations (GGDC) between strain 1 T and other members of the genus Alteromonas showed values of 70-80 %, and below 26 %, respectively. The phenotypic, phylogenetic and genomic analyses show that strain 1 T is distinct from species of the genus Alteromonas with validly published names and that it represents a novel species of the genus Alteromonas, for which the name Alteromonas fortis sp. nov. is proposed. The type strain is 1 T (= ATCC 43554 T = CIP XXXX).