Plastic has become one of the most widely manufactured materials in the world and an indispensable part of everyday life. However, a large amount of plastic waste needs to be recycled to protect the environment. One way of recycling is to use such waste as a raw material substitute. This paper evaluates the thermal and mechanical performance of eco-friendly mortars containing 0%, 5%, 10%, 15% and 20% recycled polyethylene terephthalate (PET) particles as partial replacements for sand. Several experiments were conducted to evaluate the thermal properties (i.e. thermal conductivity, thermal diffusivity and heat capacity), compressive strength, density, workability and ultrasonic pulse velocity of these mortars. Results show that replacing sand with recycled PET in cement-based mortars reduces their thermal conductivity, thereby highlighting the potential use of these mortars as energy-efficient and environmentally friendly construction materials.