Neutron diffraction studies on epitaxially strained orthorhombic (010) oriented TbMnO₃ thin films verify a theoretical prediction of an unusual magnetic phase with coexisting incommensurate and a commensurate order. Magnetic field dependent measurements of the (0 q 1) and (0 1 + q 1) Bragg reflections confirm that the magnetic intensity corresponding to an incommensurate q value has contributions from magnetic domains with both incommensurate and commensurate order. The ratio between the population of domains with commensurate and incommensurate order can be tuned by the magnetic field. In addition, the incommensurate order shows strong evidence of supporting the predicted ab cycloidal spin structure.