Due to the large amount of toxic effluents generated by the textile industry using most of synthetic dyes, natural dyes are increasingly gaining prominence, in keeping with the global trend toward prioritizing the use of cleaner technologies. However, the application of natural dyes is still restricted, mainly due to their instability in solution and poor fixation onto textile substrates. Thus, research is required to improve the performance of these dyes in order to obtain products with the characteristics desired by the industry. In this study, the potential of natural annatto dye in the dyeing of polyester (PES) fiber was analyzed and the color and stability parameters in solution were evaluated through two experiments. Initially, the stability of the natural annatto dye in solution was determined under different conditions of temperature, pH, and dye concentration and the application of this dye in conventional PES fiber dyeing was investigated. Subsequently, the influence of the use of a carrier in the dyeing of PES fiber was evaluated. The results indicated that the variables with the greatest effect on the degradation of the dye are temperature and pH. The natural annatto dye showed good interaction with the PES fiber in the conventional dyeing process, presenting good results of color yield and levels of fastness. In the dyeing processes conducted with and without a carrier, the dyeing performed in the latter provided better results, with the advantage of allowing the application of lower temperatures.