Macroalgal dissolved organic carbon (DOC) supports coastal carbon cycling and the oceanic DOC pool. Salinity fluctuates substantially in coastal environments due to natural and anthropogenic factors. Here we determined the effect of short-term salinity changes on DOC release by Sargassum fallax (Phylum Ochrophyta). Independent ramuli (~1-4 g) cut at the axes of mature individuals were incubated across a salinity gradient (4-46 ppt) for 24 h under a 12:12 light:dark cycle, seawater was sampled for DOC at 0, 12 and 24 h. Physiological assays were undertaken after the 24 h DOC experiment. DOC release (µmol C • g DW-1 • h-1) increased with decreasing salinity (F(2, 29) = 72.94, Adjusted R2 = 0.8227, p < 0.05) while net photosynthesis (µmol O2 • g • h-1) decreased (F(2, 29) = 28.23, Adjusted R2 = 0.6372, p < 0.05). Mean DOC release at 4 ppt was ~3.3 times greater in the light than the dark indicating two mechanisms; light-mediated active DOC exudation and passive release linked to osmotic stress. Hyposalinity alters the osmotic status of S. fallax and increases DOC release. This has important implications for understanding how salinity conditions encountered by macroalgae may affect their role in coastal carbon cycling.