The research on metals effect on macroalgae has been focused on Cd, Cu, Zn and Pb, and no study dealt with the effects of mercury on macroalgae growth rate. Also, the kinetic of accumulation is not well known. The main aim of this work was to assess the mercury accumulation kinetics of Ulva lactuca and its effects on growth rate. Three concentrations were tested: 5, 50 and 500 μg L−1, under static and renewal conditions and at two salinities (15 and 35), during 72 h. The mercury accumulation kinetic patterns were different according to the exposure conditions and metal concentration, but were always a very fast process (hours). Three patterns were established based on the models substrate inhibition, linear regression and Michaelis–Menten equation. Statistical differences in the mercury accumulated were recorded depending on the salinity values and exposure conditions, being higher at salinity 15 and at renewal tests, corresponding to the lower relative growth rates. The lowest mercury concentration did not have an effect on relative growth rate, while the others caused an accentuated inhibition at 24 h. The highest concentration was toxic to algae causing its death before 48 h. Under controlled conditions, the U. lactuca’s high and fast ability for mercury accumulation could be useful for phytoremediation and for industrial wastewaters treatment.