The metavalently bonded SnTe has gathered enormous attention recently as an environmentally friendly alternative to PbTe. While some of the issues with SnTe, including its small bandgap and large valence band offset, have largely been resolved, its relatively high lattice thermal conductivity (κl) has remained a matter of concern. Here, we show that the temperature-induced vacancy migration in SnTe results in the growth of SnTe nanoparticles in the intergrain region. The enhanced grain boundary scattering due to this led to a highly reduced κl and increased carrier mobility, enhancing the zT of our SnTe by almost 70% over the zT of various SnTe ingot samples from this and several previous studies. The validity of this approach was further confirmed for a 3% Ag-doped SnTe sample, a composition well-investigated in the past. The simplicity and effectiveness of our approach enhance the prospect of SnTe for practical applications.