The integration of magnetic material with semiconductors has been fertile ground for fundamental science as well as of great practical interest toward the seamless integration of information processing and storage. We create van der Waals heterostructures formed by an ultrathin ferromagnetic semiconductor CrI₃ and a monolayer of WSe₂. We observe unprecedented control of the spin and valley pseudospin in WSe₂, where we detect a large magnetic exchange field of nearly 13 T and rapid switching of the WSe₂ valley splitting and polarization via flipping of the CrI₃ magnetization. The WSe₂ photoluminescence intensity strongly depends on the relative alignment between photoexcited spins in WSe₂ and the CrI₃ magnetization, because of ultrafast spin-dependent charge hopping across the heterostructure interface. The photoluminescence detection of valley pseudospin provides a simple and sensitive method to probe the intriguing domain dynamics in the ultrathin magnet, as well as the rich spin interactions within the heterostructure.