Five novel Gram-reaction-negative aerobic marine bacterial strains with DNA G+C contents <50 mol% were isolated from the seagrass Posidonia oceanica. 16S rRNA sequence analysis indicated that they belonged to the genus Marinomonas. Major fatty acid compositions, comprising C₁₀:₀ 3-OH, C₁₆:₀, C₁₆:₁ω7c and C₁₈:₁ω7c, supported the affiliation of these strains to the genus Marinomonas. Strains IVIA-Po-14b(T), IVIA-Po-145(T) and IVIA-Po-155(T) were closely related to Marinomonas pontica 46-16(T), according to phylogenetic analysis. However, DNA-DNA hybridization values <35 % among these strains revealed that they represented different species. Further differences in the phenotypes and minor fatty acid compositions were also found among the strains. Another two strains, designated IVIA-Po-181(T) and IVIA-Po-159(T), were found to be closely related to M. dokdonensis DSW10-10(T) but DNA-DNA relatedness levels <40 % in pairwise comparisons, as well as some additional differences in phenotypes and fatty acid compositions supported the creation of two novel species. Accordingly, strains IVIA-Po-14b(T )( = CECT 7730(T) = NCIMB 14671(T)), IVIA-Po-145(T) ( = CECT 7377(T) = NCIMB 14431(T)), IVIA-Po-155(T) ( = CECT 7731(T) = NCIMB 14672(T)), IVIA-Po-181(T) ( = CECT 7376(T) = NCIMB 14433(T)) and IVIA-Po-159(T) ( = CECT 7732(T) = NCIMB 14673(T)) represent novel species, for which the names Marinomonas alcarazii sp. nov., Marinomonas rhizomae sp. nov., Marinomonas foliarum sp. nov., Marinomonas posidonica sp. nov. and Marinomonas aquiplantarum sp. nov. are proposed, respectively.