WEAKLY bound van der Waals molecules, consisting of atoms or molecules (such as rare gases, H2 and N2) interacting through van der Waals forces, can be formed in the gas phase1. The existence of similar weakly bound stoichiometric compounds in condensed phases has remained an open question as the components usually form separate pure phases or solid solutions2. Here we report on a detailed study of the helium-nitrogen system in a diamond-anvil cell using synchrotron X-ray diffraction, Raman scattering and optical microscopy. We find that high pressure stabilizes the formation of a stoichiometric, solid van der Waals compound of composition He(N2)11. Because of the relatively simple interactions involved, this solid may exemplify a novel class of van der Waals compounds that are formed only at high pressures. Van der Waals chemistry in simple molecular systems may be important for understanding the structure and properties of the interiors of the outer planets and their satellites, where pressures are high and such components may be abundant3-5.