A new determination of the decay rate of the negative ion of positronium (${\mathrm{Ps}}^{$-${}}$), using a beam-foil method and a stripping-based detection technique, is reported. The measured result of $$Γ${}=2.089(15)\text{ }\text{ }{\mathrm{ns}}^{$-${}1}$ is a factor of 6 more precise than the previous experimental value of $$Γ${}=2.09(9)\text{ }\text{ }{\mathrm{ns}}^{$-${}1}$, and is in excellent agreement with the theoretical value of $$Γ${}=2.086(6)\text{ }\text{ }{\mathrm{ns}}^{$-${}1}$.