We report on the dielectric properties of epitaxial [001] and [100]-textured thin films of antiferromagnetic orthorhombic YMnO3 and their variation under compressive epitaxial strain. It is found that weakly strained YMnO3 films are ferroelectric with a polarization along c-axis switchable by 90 degrees by an external magnetic field. When reducing film thickness and increasing epitaxial strain, the ferroelectric order is progressively suppressed. Analysis of structural, dielectric and magnetic data indicates that suppression of ferroelectricity when reducing thickness is accompanied by an enlarged ratio a/b of the in-plane cell parameters of the orthorhombic structure and the appearance of a net magnetization. All results can be well described by considering the multiferroic phase diagram of orthorhombic manganites. ; Comment: Accepted: Phase Transitions