Topological insulators were originally discovered in condensed matter systems. Recently this concept has been transferred to bosonic systems such as photons and phonons, which has provided methods for constructing non-trivial states. In principle, topology protects propagation of phonon against backscattering, but not against loss, which has remained limited to the dB/cm-level for phonon waveguides, be they topological or not. On-chip phononic waveguides with propagation losses due to dissipation of ~3 dB/km at room temperature is reported, which is orders of magnitude below any previous chip-scale devices. This is realized by the combination of advanced dissipation engineering, in particular the recently introduced method of soft-clamping, with the concept of a valley-Hall topological insulator for phonons. The low losses enable high-resolution ultrasound spectroscopy, and thus for the first time allow to accurately quantify backscattering protection in a topological phonon waveguide. We infer that phonons follow a sharp, 120°-bend with a 99.99%-probability instead of being scattered back, and less than one phonon in a million is lost. We also demonstrated that our phononic system has strong nonlinearities that allows large parametric gain. Our work will inspire new research directions on ultralow loss phononic waveguides, and provides a clean bosonic system for investigating topological protection and non-Hermitian topological physics.