<p>In recent years, although two-dimensional (2D) Janus transition metal dichalcogenide (TMDC) monolayers have gradually been applied to electronic devices owing to their distinctive physical properties, their phonon thermal transport properties remain unclear. Here, we systematically explore the phonon thermal properties of six types of 2D Janus TMDC monolayers MXY (M&#xa0;=&#xa0;Mo, S; X, Y&#xa0;=&#xa0;S, Se, Te) by performing the first-principles calculations. After considering the size effect, the phononic thermal conductivities (<i>k</i>_ph) of these monolayers have size-dependency up to more than 10–100&#xa0;<i>μ</i>m. Moreover, the atomic mass dependence of the <i>k</i>_ph is observed, and the Janus TMDC monolayers with a Te element have smaller <i>k</i>_ph. Furthermore, in monolayers with small system size, phonon–boundary scattering has a greater impact on <i>k</i>_ph, leading to a weakened temperature dependence of <i>k</i>_ph. Finally, based on the phonon behaviors, we can prove that MSSe monolayers have the largest <i>k</i>_ph owing to their higher phonon heat capacity, group velocity, and lifetime, whereas the competition between the three parameters leads to the <i>k</i>_ph being close to each other for MSTe and MSeTe monolayers. Our research offers a necessary study of phonon thermal transport abilities in 2D Janus TMDC monolayers.</p> Graphical Abstract <p></p>