The explosive development of wearable electronic devices places new requirements on the flexibility of component materials. As one of the key energy harvesting sources for wearable electronic devices, thermoelectric (TE) generators convert waste heat into electric energy and can operate without producing any noise or requiring any external stimuli (e.g., light, or mechanical motion). However, conventional materials developed for TE possess very limited flexibility and their performances remain low towards practical applications. TE composites represent an effective way to "reshape" the properties of the pristine materials further enhancing their performances. Among various fillers for TE composites, insulating polymers are now becoming one of the promising candidates since they possess excellent flexibility and could efficiently decouple the dependence between electrical conductivity, Seebeck coefficient, and thermal conductivity. Currently, there is a relatively limited knowledge of this new category of TE composites and lots of research progress on them have already demonstrated their potentials for practical applications. In this review, we will discuss this new type of TE composites from a unique perspective and attempt to summarize the basic properties, design rules, and the latest research progresses on them. A brief discussion on their potential applications, remaining challenges, and future prospects will also be included in the last section.