This study compared the biosorption of tetracycline, an antibiotic commonly used to treat bacterial infections, in aqueous solution under various conditions using two brown seaweeds commonly found in Hong Kong waters—Pachydictyon coriaceum and Sargassum hemiphyllum. Two environmental effects (temperature and shaking speed) and two chemical effects (pH and salinity) were investigated to determine the optimal conditions for sorption of tetracycline by biomass. It was found that the maximum biosorption capacity (q max) of tetracycline by the two types of seaweed was generally higher at lower temperature (15 °C) and higher shaking speed (250 rpm). The sorption performances of P. coriaceum and S. hemiphyllum were better in slightly acidic solution (pH 3), with q max around 9 mg/g for P. coriaceum. Higher salinity (100 mM NaCl) reduced the sorption ability of both brown seaweeds by reducing the solubility of the aqueous tetracycline. It was found that S. hemiphyllum could tolerate and had higher sorption in a slightly saline solution (50 mM NaCl), while P. coriaceum performed better without the presence of NaCl. This study provides crucial information for achieving optimal sorption of aqueous tetracycline using P. coriaceum over S. hemiphyllum as an effective biomass for removing antibiotics in wastewater.