Polysaccharides from marine algae exhibit beneficial biological activities. In this study, we examined the effect of polysaccharides from Codium fragile (PCF) on prebiotic and anti-obesity activity. PCF increases the growth of specific beneficial microbial populations with concomitant decrease in pathogenic microbes. Further, total phenolic content (TPC), total flavonoid content (TFC), and DPPH radical scavenging activity (DPPH activity) after fermentation with PCF as carbon source were higher than for the glucose as control. Moreover, PCF inhibited adipocyte differentiation by inducing differentiation-related factors when the induction of 3T3-L1 preadipocytes into adipocytes was induced. Therefore, we suggest that PCF can be used as prebiotic material with anti-obesity for human health.