Superconductivity originates from the formation of bound (Cooper) pairs of electrons that can move through the lattice without resistance below the superconducting transition temperature Tc . While electron Cooper pairs in most superconductors form anti-parallel spin-singlets with total spin S = 0, they can also form parallel spin-triplet Cooper pairs with S = 1 and an odd parity wavefunction. Spin-triplet pairing is important because it can host topological states and Majorana fermions relevant for quantum computation. Since spin-triplet pairing is usually mediated by ferromagnetic (FM) spin fluctuations, uranium based materials near a FM instability are considered ideal candidates for realizing spin-triplet superconductivity. Indeed, UTe2, which has a Tc=1.6 K, has been identified as a candidate for chiral spin-triplet topological superconductor near a FM instability, although it also has antiferromagnetic (AF) spin fluctuations. Here we use inelastic neutron scattering (INS) to show that superconductivity in UTe2 is coupled with a sharp magnetic excitation, termed resonance, at the Brillouin zone (BZ) boundary near AF order. Since the resonance has only been found in spin-singlet unconventional superconductors near an AF instability, its discovery in UTe2 suggests that AF spin fluctuations may also induce spin-triplet pairing or that electron pairing in UTe2 has a spin-singlet component. [C. R. Dun et al., Nature (in press) 2021; Phys. Rev. Lett. 125, 237003 (2020)].