Salinity is among the eminent environmental factors besides temperature, nutrients, and light that affect the growth and distribution of macroalgae. An experiment under laboratory conditions was conducted to determine the effect of salinity on the growth of the enigmatic green macroalgae, Caulerpa macrodisca . Simulating the common in situ salinity conditions around the sampling stations, five salinity ranges (17, 21, 24, 27, and 31 ppt) were tested in this experiment. The C. macrodisca fragments were cultured in a flask with controlled temperature and light intensity under a 12:12 photoperiod. The following growth responses were measured weekly: wet biomass (g), disc diameter (cm), stolon length (cm), and frond length (cm). In the experimental cultures, growth (increase in wet biomass, disc diameter, stolon length, and frond length) was calculated as the relative growth rate (RGR), expressed as a percentage of daily growth. The results showed that salinity had a significant effect (p<0.05)on the RGR of C. macrodisca. Maximal RGR of wet biomass ( 5.08 ± 0.04 % day−1 ) was achieved at 27 ppt, while the negative growth rate was obtained at 17 ppt, with discolouration and progressive deterioration of C. macrodisca being observed. These results are in line with C. macrodisca’s native habitat in the waters of Menumbok , where it is active throughout its yearly cycle near regions with an average salinity of 23 to 30 ppt. According to the study, the physiological responses derived from the results are significant in establishing a baseline for the ecology of the habitat for C. macrodisca. This work is important for examining potential environmental influences on the development of the mysterious Caulerpa in tropical estuaries.