  We present APO and Gemini time-series photometry of WD
J004917.14$-$252556.81, an ultramassive DA white dwarf with $T_{\rm eff} =
13020$ K and $\log{g} = 9.34$. We detect variability at two significant
frequencies, making J0049$-$2525 the most massive pulsating white dwarf
currently known with $M_\star=1.31~M_{\odot}$ (for a CO core) or
$1.26~M_{\odot}$ (for an ONe core). J0049$-$2525 does not display any of the
signatures of binary mergers, there is no evidence of magnetism, large
tangential velocity, or rapid rotation. Hence, it likely formed through single
star evolution and is likely to have an ONe core. Evolutionary models indicate
that its interior is $\gtrsim99$% crystallized. Asteroseismology offers an
unprecedented opportunity to probe its interior structure. However, the
relatively few pulsation modes detected limit our ability to obtain robust
seismic solutions. Instead, we provide several representative solutions that
could explain the observed properties of this star. Extensive follow-up
time-series photometry of this unique target has the potential to discover a
significant number of additional pulsation modes that would help overcome the
degeneracies in the asteroseismic fits, and enable us to probe the interior of
an $\approx1.3~M_{\odot}$ crystallized white dwarf.
