Abstract We present the direct-imaging discovery of a giant planet orbiting the young star AF Lep, a 1.2 M _⊙ member of the 24 ± 3 Myr β 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 ≈340 mas, which exhibits orbital motion at the 6σ level over the course of 13 months. A joint orbit fit yields precise constraints on the planet’s dynamical mass of 3.2 − 0.6 + 0.7 M _Jup, semimajor axis of 8.4 − 1.3 + 1.1 au, and eccentricity of 0.24 − 0.15 + 0.27 . AF Lep hosts a debris disk located at ∼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 − 9 + 11 ° ) and orbital inclination (i _o = 50 − 12 + 9 ° ) 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.