A levitated nanomechanical oscillator under ultra-high vacuum (UHV) is highly isolated fromits environment. It has been predicted that this isolation leads to very low mechanical dissipationrates. However, a gap persists between predictions and experimental data. Here, we levitate a silicananoparticle in a linear Paul trap at room temperature, at pressures as low as 7 × 10−11 mbar. Wemeasure a dissipation rate of 2π ×69(22) nHz, corresponding to a quality factor exceeding 1010, morethan two orders of magnitude higher than previously shown. A study of the pressure dependence ofthe particle’s damping and heating rates provides insight into the relevant dissipation mechanisms.