  Measuring the obliquities of stars hosting giant planets may shed light on
the dynamical history of planetary systems. Significant efforts have been made
to measure the obliquities of FGK stars with hot Jupiters, mainly based on
observations of the Rossiter-McLaughlin effect. In contrast, M dwarfs with hot
Jupiters have hardly been explored, because such systems are rare and often not
favorable for such precise observations. Here, we report the first detection of
the Rossiter-McLaughlin effect for an M dwarf with a hot Jupiter, TOI-4201,
using the Gemini-North/MAROON-X spectrograph. We find TOI-4201 to be
well-aligned with its giant planet, with a sky-projected obliquity of
$\lambda=-3.0_{-3.2}^{+3.7}\ ^{\circ}$ and a true obliquity of
$\psi=21.3_{-12.8}^{+12.5}\ ^{\circ}$ with an upper limit of $40^{\circ}$ at a
95% confidence level. The result agrees with dynamically quiet formation or
tidal obliquity damping that realigned the system. As the first hot Jupiter
around an M dwarf with its obliquity measured, TOI-4201b joins the group of
aligned giant planets around cool stars ($T_{\rm eff}<6250\ K$), as well as the
small but growing sample of planets with relatively high planet-to-star mass
ratio ($M_p/M_\ast\gtrsim 3\times 10^{-3}$) that also appear to be mostly
aligned.
