We have used high-energy inelastic neutron scattering to detect optical magnons directly in antiferromagnetic YBa2Cu3O6.2. The optical magnon gap is 67+/-5 meV. This implies an intrabilayer superexchange constant perpendicular to the CuO2 layers of J(perpendicular to) = 0.08 J(parallel to) where J(parallel to) is the in-plane nearest-neighbor superexchange constant.