Transcripts and enzyme activities of antioxidative enzymes were increased by hypersalinity (90‰) in a marine macroalga, Ulva fasciata Delile (Lu et al. 2006, Sung et al. 2009). This study examined the effects of polyamines (PAs) on the induction of hypersalinity tolerance through the modulation of expression of antioxidative defense enzymes. Incubation of U. fasciata grown under 30‰ in the presence of putrescine (Put), spermidine (Spd), or spermine (Spm) (1 mM) for 12 h increased internal PA contents prior to 90‰ treatment. Spd or Spm pretreatments reduced H2O2 accumulation and lipid peroxidation during 90‰ treatment and improved the recovery growth rate after transfer from 90‰ to 30‰. Increases in iron superoxide dismutase (FeSOD; EC 184.108.40.206) activity and transcript levels observed under 90‰ were further increased by Spd and Spm pretreatments, while Put pretreatment had no effect. Increases in MnSOD activity and transcript levels observed under 90‰ were enhanced by Spd and Put pretreatment. An observed increase in catalase (CAT; EC 220.127.116.11) activity and transcript levels under 90‰ was not affected by Spd and Spm pretreatments but was inhibited by Put pretreatment. Observed increases in ascorbate peroxidase (APX; EC 18.104.22.168) activity and transcript levels under 90‰ were inhibited by Put, Spd, and Spm pretreatments. In conclusion, Spd and Spm treatment affords U. fasciata protection against hypersalinity through the up-regulation of FeSOD gene expression, thereby alleviating oxidative damage.