Large- scale production and use of silver nanoparticles (AgNPs) may lead to their release into the marine environment, which can consequently lead to the death of various marine organisms. Previous research has proven the toxicity of AgNPs to marine invertebrates and phytoplankton, but there are scarcely any data available on their effect on marine macroalgae. Marine macroalgae are a very important part of the marine ecosystem as they enrich the marine environment with oxygen, are important food for herbivorous species of organisms and provide a habitat and hiding place for different species of vertebrates and invertebrates. The aim of this work was to examine how different particle sizes modulates the toxicity of silver nanoparticles to the red marine macroalgae Laurencia obtusa. The toxicity of AgNPs was investigated using lipid peroxidation and hydrogen peroxide content as indicators of negative stress. The thalli of L. obtusa were exposed to AgNPs, of sizes of 20 and 60 nm, in concentrations of 1, 10 and 100 μg/mL for a period of 24 hours. The results of the lipid peroxidation test showed the highest toxicity from Ag60 at a concentration of 100 μg/mL, while Ag20 did not significantly affect toxicity towards L. obtusa. Hydrogen peroxide content also indicated greater toxicity of Ag60, but at the lowest tested concentration. In both tests, AgNPs typically showed greater lipid peroxidation and hydrogen peroxide generation than Ag+ ions. Furthermore, larger AgNPs were found to have a greater impact on toxicity. Ultimately, it is necessary to carry out further research to determine with greater certainty the impact of AgNPs on marine macroalgae using a wider range of endpoints.