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Polymyxin-producing bacteria within the <i>Paenibacillus polymyxa</i> complex have broad-spectrum activities against fungi and bacteria. Their antibacterial activities against soft rot <i>Dickeya</i> and <i>Pectobacterium</i> phytopathogens containing multiple polymyxin-resistant genes were not clear. Here, we selected nine strains within the <i>P. polymyxa</i> complex having broad-spectrum antagonistic activities against phytopathogenic fungi and a polymyxin-resistant <i>D. dadantii</i> strain causing stem and root rot disease of sweet potato and did antagonistic assays on nutrient agar and sweet potato tuber slices. These strains within the <i>P. polymyxa</i> complex showed clear antagonistic activities against <i>D. dadantii</i> in vitro and in vivo. The most effective antagonistic strain <i>P. polymyxa</i> ShX301 showed broad-spectrum antagonistic activities against all the test <i>Dickeya</i> and <i>Pectobacterium</i> strains, completely eliminated <i>D. dadantii</i> from sweet potato seed tubers, and promoted the growth of sweet potato seedlings. Cell-free culture filtrate of <i>P. polymyxa</i> ShX301 inhibited <i>D. dadantii</i> growth, swimming motility, and biofilm formation and disrupted <i>D. dadantii</i> plasma membranes, releasing nucleic acids and proteins. Multiple lipopeptides produced by <i>P. polymyxa</i> ShX301 may play a major role in the bactericidal and bacteriostatic actions. This study clarifies that the antimicrobial spectrum of polymyxin-producing bacteria within the <i>P. polymyxa</i> complex includes the polymyxin-resistant <i>Dickeya</i> and <i>Pectobacterium</i> phytopathogens and strengthens the fact that bacteria within the <i>P. polymyxa</i> complex have high probability of being effective biocontrol agents and plant growth promoters.