Marine macroalgae are an excellent source of biologically active compounds. Among tested 8 different marine macroalgal species, we intensively studied the antibacterial activity of the methanol extract of four selected algal species; Ulva lactuca, Caulerpa racemosa, Acanthophora spicifera and Sargassum dentifolium against Ralstonia solanacearum, the causal agent of bacterial wilt. In vitro assay showed that U. lactuca had the highest antibacterial activity while Sargassum dentifolium showed the lowest effect. In contrast, A. spicifera extract was the most disease suppressive in the in vivo study. The suppressive effects of extracts were generally low in sandy soil compared to a clay one. The interrelation between growth inhibition of R. solanacearum, wilt suppression and phenolic constitutions in algal extracts were investigated. Results showed that gallic and coumaric acids were correlated with inhibition of the pathogen in vitro while hydroxybenzoic, chlorogenic, vanillic, salicylic and ferulic acids were in vivo correlated with disease suppression under greenhouse conditions. Use of algal extracts, as a strong antibacterial and antioxidant, can be employed as a part of an integrated program for controlling of plant diseases. Further investigations are needed on the feasibility of the application.