  We present the direct imaging discovery of a giant planet orbiting the young
star AF Lep, a 1.2 $M_{\odot}$ member of the 24 $\pm$ 3 Myr $\beta$ Pic moving
group. AF Lep was observed as part of our ongoing high-contrast imaging program
targeting stars with astrometric accelerations between Hipparcos and Gaia that
indicate the presence of substellar companions. Keck/NIRC2 observations in $L'$
with the Vector Vortex Coronagraph reveal a point source, AF Lep b, at
${\approx}340$ mas which exhibits orbital motion at the 6-$\sigma$ level over
the course of 13 months. A joint orbit fit yields precise constraints on the
planet's dynamical mass of 3.2$^{+0.7}_{-0.6}$ $M_\mathrm{Jup}$, semi-major
axis of $8.4^{+1.1}_{-1.3}$ au, and eccentricity of $0.24^{+0.27}_{-0.15}$. AF
Lep hosts a debris disk located at $\sim$50 au, but it is unlikely to be
sculpted by AF Lep b, implying there may be additional planets in the system at
wider separations. The stellar inclination ($i_* = 54^{+11}_{-9} {}^\circ$) and
orbital inclination ($i_o = 50^{+9}_{-12} {}^\circ$) are in good agreement,
which is consistent with the system having spin-orbit alignment. AF Lep b is
the lowest-mass imaged planet with a dynamical mass measurement and highlights
the promise of using astrometric accelerations as a tool to find and
characterize long-period planets.

Earth and Planetary Astrophysics;Solar and Stellar Astrophysics