In recent years, the accumulation of microplastics and nanoplastics in the aquatic environment has increased, which threatens water security and the ecosystem. Microplastics and nanoplastics should be detected and removed from the aquatic environment to ensure the safety and security of the aquatic ecosystem. Thermal analysis is a promising technique to detect microplastics and nanoplastics in the aquatic environment. This study focuses on analyzing the influence of microplastics on the thermal conductivity of artificial seawater. The thermal conductivity of artificial seawater and PE (polyethylene) microplastics spiked with artificial seawater at the concentrations of 120 and 160 mg/L were measured using a thermal conductivity meter (Flucon LAMBDA thermal conductivity meter) with a temperature range of 28–70℃. Scanning Electron Microscope (SEM) analysis was conducted on pristine PE microplastics and PE microplastics exposed to thermal conductivity analysis. Thermal conductivity measurements confirmed that PE microplastics lower the thermal conductivity of artificial seawater in the temperature range 28–40℃ due to their thermal insulation property. The thermal conductivity of artificial seawater was increased in the temperature range of 40–70℃ due to the Brownian motion of smaller-sized particles. SEM images confirmed that PE microplastics were fragmented when the temperature was above 40℃. These fragmented particles increased the thermal conductivity of artificial seawater. This study demonstrates that measuring thermal conductivity could be a novel and low-cost detection method for microplastics and nanoplastics in seawater.