  The alignment of planetary orbits with respect to the stellar rotation
preserves information on their dynamical histories. Measuring this angle for
young planets help illuminate the mechanisms that create misaligned orbits for
older planets, as different processes could operate over timescales ranging
from a few Myr to a Gyr. We present spectroscopic transit observations of the
young exoplanet V1298 Tau b; we update the age of V1298 Tau to be $28\pm4$ Myr
based on Gaia EDR3 measurements. We observed a partial transit with Keck/HIRES
and LBT/PEPSI, and detected the radial velocity anomaly due to the
Rossiter-McLaughlin effect. V1298 Tau~b has a prograde, well-aligned orbit,
with $\lambda = 4_{-10}^{+7 \circ}$. By combining the
spectroscopically-measured $v\sin i_{\star}$ and the phtometrically-measured
rotation period of the host star we also find that the orbit is aligned in 3D,
$\psi = 8_{-7}^{+4 \circ}$ deg. Finally, we combine our obliquity constraints
with a previous measurement for the interior planet V1298 Tau c to constrain
the mutual inclination between the two planets to be
$i_{\mathrm{mut}}=0^{\circ} \pm 19^{\circ}$. This measurements adds to the
growing number of well-aligned planets at young ages, hinting that
misalignments may be generated over timescales of longer than tens of Myr. The
number of measurements, however, is still small, and this population may not be
representative of the older planets that have been observed to date. We also
present the derivation of the relationship between $i_{\mathrm{mut}}$,
$\lambda$, and $i$ for two planets.
