  We report the discovery of TOI-4641b, a warm Jupiter transiting a rapidly
rotating F-type star with a stellar effective temperature of 6560 K. The planet
has a radius of 0.73 $R_{Jup}$, a mass smaller than 3.87 $M_{Jup}$ $(3\sigma)$,
and a period of 22.09 days. It is orbiting a bright star (V=7.5 mag) on a
circular orbit with a radius and mass of 1.73 $R_{\odot}$ and 1.41 $M_{\odot}$.
Follow-up ground-based photometry was obtained using the Tierras Observatory.
Two transits were also observed with the Tillinghast Reflector Echelle
Spectrograph (TRES), revealing the star to have a low projected spin-orbit
angle ($\lambda$=$1.41^{+0.76}_{-0.76}$ degrees). Such obliquity measurements
for stars with warm Jupiters are relatively few, and may shed light on the
formation of warm Jupiters. Among the known planets orbiting hot and
rapidly-rotating stars, TOI-4641b is one of the longest-period planets to be
thoroughly characterized. Unlike hot Jupiters around hot stars which are more
often misaligned, the warm Jupiter TOI-4641b is found in a well-aligned orbit.
Future exploration of this parameter space can add one more dimension to the
star-planet orbital obliquity distribution that has been well-sampled for hot
Jupiters.
