The rotational excitation of NH3 in collisions with He is investigated at a collision energy of 140 meV using the method of counterpropagating pulsed molecular beam scattering in combination with resonance-enhanced multiphoton ionization detection. Initial state preparation is achieved through the adiabatic expansion of NH3 seeded in Ne. The scattered intensity exhibits a strong dependence on the direction of the laser polarization relative to the molecular beam axis. From the polarization dependence, degeneracy-averaged state-resolved integral and differential cross sections as well as their quadrupole moments could be determined for many final states in both modifications. The degeneracy-averaged cross sections are in reasonable agreement with the results of an earlier calculation based on a semiempirical potential surface. Final states, corresponding to large energy transfer, show strong negative collision-induced alignment for backward scattering. For states representing small energy transfer, we find a smaller degree of alignment which changes from positive to negative values with increasing scattering angle. This general behavior is consistent with a strong propensity for transitions with Delta m << Delta j with respect to the direction of linear momentum transfer.