We demonstrate that the KPbmSbTe2+m system (PLAT-m for tellurium, antimony, lead potassium, m = 19−21) of materials exhibits high thermoelectric performance. Samples with compositions K1−xPbm+δSb1+γTem+2 were prepared using several combinations of x, δ, γ and m and their thermoelectric properties were investigated in the temperature range of 300 − 800 K. All K1−xPbm+δSb1+γTem+2 samples exhibited n-type conduction over the measured temperature range. Their lattice thermal conductivities were found to be significantly reduced when compared to PbTe and even AgPbmSbTem+2. For example, for K0.95Pb20Sb1.2Te22 a lattice thermal conductivity as low as 0.4 W/(m·K) was estimated at 650 K (based on a Lorenz number of 1.25 × 10−8 W·Ω/K2). High resolution transmission electron microscopy on several samples revealed a widely dispersed nanoscale particle with varying size and shape endotaxially embedded inside a PbTe-rich matrix which is believed to be responsible for the reduced lattice thermal conductivity of K1−xPbm+...