Thermochromic materials, are one of the most interesting smart materials in the textile industry, are dyes or pigments that change colour when exposed to different temperatures. Nowadays, they can enrich the visual properties of textile materials and provide functionality to technological properties. Furthermore, they can be used as body thermometers due to their ability to change colour with temperature. Thermochromic textiles are commonly used in sports clothing (such as swimming and ski suits), laboratory gloves, work clothes, and clothes with temperature indicators. These textiles gain their thermochromic properties through the application of chromic-based pigment to the final fabric surface using finishing processes like dyeing and printing. However, the resulting functional properties are usually not permanent. Some studies that chromic pigment added synthetic yarns such as only polyethylene, polyamide, and polypropylene polymers in the melt spinning process were found in the literature. In this study, we aim to produce polyester multifilament yarn with adding thermochromic additive at a rate of 1.0% or 1.6% using the melt spinning method. Moreover, mechanical, physical and thermal analyzes were performed to understand how the content of the thermochromic additive effects the properties of multifilament yarns such as color, tenacity, elongation at break, linear density, melting point, etc. It has been observed that thermochromic additive changes the mechanical, physical and thermal properties of polyester multifilament and polyester yarns with thermochromic pigment additives at a ratio of 1% and above exhibit colour-changing properties.