We develop a phenomenological mean-field theory describing the hidden-order phase in URu2Si2 as a nematic of the B1g representation staggered along the c axis. Several experimental features are reproduced by this theory: The topology of the temperature-pressure phase diagram, the response of the elastic modulus (C11-C12)/2 above the transition at ambient pressure, and orthorhombic symmetry breaking in the high-pressure antiferromagnetic phase. In this scenario, hidden order is characterized by broken rotational symmetry that is modulated along the c axis, the primary order of the high-pressure phase is an unmodulated nematic, and the triple point joining those two phases with the high-temperature paramagnetic phase is a Lifshitz point. © 2020 American Physical Society.