In recent years, microplastics in oceans have become a serious environmental problem and the focus of attention. In the present study, the sorption of TBC and HBCDs by microplastics in simulated seawater is examined. The effects of particle size, temperature, salinity, and concentration on the adsorption of TBC and HBCDs by microplastics are studied. Results indicate that the first-order adsorption kinetic model is more suitable than the pseudo-second-order kinetic model to describe adsorption. The equilibrium adsorption times are 15 h and 10 h for TBC and HBCDs, respectively. The adsorption capacity increases with the decrease in particle size. The adsorption capacity gradually increases at first and then decreases with the increase in salinity and temperature. The maximum adsorption capacity is at 15 °C and 14% salinity. Compared with the linear and Freundlich models, the Langmuir model is more suitable; this indicates that the main adsorption mechanism might be chemical adsorption.