This Element shows Open theism as a monotheist model of God according to which the future is objectively open-ended, not just from the finite perspective of creation, but from God's perspective as well. This Element has three main sections. The first carefully defines open theism, distinguishes its major variants, compares it to other monotheistic models, and summarizes its history. The second develops biblical and philosophical arguments for open theism against its main rivals, emphasizing a novel philosophical argument that a causally open future must also be ontically, alethically, epistemically, and providentially open as well. The third responds to common objections against open theism related to perfect being theology, the ethics of risk-taking, biblical prophecy, and theological tradition.