Numerous studies on poly gamma-d-glutamicacid (gamma-PGA) production have investigated terrestrial renewable sources for reducing production costs, but there are no studies using waste marine resources so far. We aimed to develop a cost-effective production method of gamma-d-PGA by Bacillus sp. SJ-10 using green macroalgae (Ulva sp.) as a major substrate without hydrolysis pretreatment. The SJ-10 was shown to not only cause immediate tissue degradation of the Ulva membrane but also grew well as a sole substrate. The gamma-d-PGA yield was 6.29 +/- 0.34 g/L under optimized conditions via the response surface method, and the produced gamma-d-PGA had a thermal decomposition temperature of 310 degrees C and molecular weight of 250-1780 kDa. The calculated cost efficiency for the final yield was 32% when compared with complex media. Therefore, the present study provided a strategy for promoting an ecofriendly and cost-effective means to produce gamma-d-PGA via a marine renewable resource.