SummaryPolyamines (PAs), such as diamine putrescine (PUT), triamine spermidine (SPD) and tetraamine spermine (SPM) have been related to environmental stress, including salt stress. A marine red macrophyte alga Grateloupia doryphora (Montagne) Howe was used to investigate the role of PAs during acclimation to moderate hyposaline conditions (incubation 24 h in 18 psu seawater as compared to 36 psu of natural seawater). The results obtained showed that a moderate hyposaline shock caused an increase in the free fraction of PUT, SPD and SPM, mainly due to a decrease in TGase activity, together with an apparent increase in the L-arginine dependent PAs synthesis (ODC and arginase decreased, and ADC slightly increased). The photosynthetic rate increased in thalli when exposed to free SPD at 18 psu, but it did not increase at 36 psu.