  The atomic mass of uranium-238 has been determined to be
$238.050\,787\,618(15)\,\text{u}$, improving the literature uncertainty by two
orders of magnitude. It is obtained from a measurement of the mass ratio of
$^{238}$U$^{47+}$ and $^{132}$Xe$^{26+}$ ions with an uncertainty of $3.5\times
10^{-12}$. The measurement was carried out with the Penning-trap mass
spectrometer \textsc{Pentatrap} and was accompanied by a calculation of the
binding energies $E_{\text{U}}$ and $E_{\text{Xe}}$ of the 47 and 26 missing
electrons of the two highly charged ions, respectively. These binding energies
were determined using an \textit{ab initio} multiconfiguration
Dirac-Hartree-Fock (MCDHF) method to be $E_{\text{U}} = 39\,927(10)\,\text{eV}$
and $E_{\text{Xe}} = 8\,971.2(21)\,\text{eV}$. The new mass value will serve as
a reference for high-precision mass measurements in the heavy mass region of
the nuclear chart up to transuranium nuclides.
