We report a thorough experimental study on the microstructure, thermal behavior and thermoelectric properties of the amorphous composition Cu15As30Te55 and the glass–ceramics related-compounds synthesized by using the Spark Plasma Sintering (SPS) technique. Varying the conditions of the SPS process enables the synthesis of composite glassy-crystalline samples with different crystal/glass ratios. Such treatments result in complex microstructures composed of large glassy domains where nanocrystals of the metastable β-As2Te3 phase are embedded. These domains are separated by regions of the dendritic crystalline phase surrounded by a Cu-rich glassy matrix. The presence of β-As2Te3, confirmed by both powder X-ray diffraction and scanning electron microscopy, suggests that pressure and/or internal stresses play an important role in stabilizing this phase. This conclusion is further supported by neutron thermodiffraction experiments revealing a sharp crossover from the β-As2Te3 to the stable α-As2Te3 phase at temperatures below that of the SPS treatment. Transport properties measurements show that the presence of a crystalline fraction significantly lowers the electrical resistivity by four orders of magnitude. However, the probable intrinsic n-type behavior of β-As2Te3 has a detrimental influence on the thermopower values. Even though the partial crystallization of the glassy matrix leads to an increase in the thermal conductivity, the measured values remain on the order of 1 W m−1 K−1 at 300 K. Besides an overall increase in the dimensionless figure of merit ZT, our results demonstrate that the partial crystallization of an amorphous matrix is an efficient tool to tune the electrical resistivity over several orders of magnitude while maintaining low thermal conductivity values.