We consider an agent that is assigned with a temporal logic task in an environment whose semantic representation is only partially known. We represent the semantics of the environment with a set of state properties, called atomic propositions. The agent holds a probabilistic belief over the atomic propositions and updates it as new sensory measurements arrive. The goal is to design a policy for the agent that realizes the task with high probability. We develop a planning strategy that takes the semantic uncertainties into account and by doing so provides probabilistic guarantees on the task success. Furthermore, as new data arrive, the belief over the atomic propositions evolves and, subsequently, the planning strategy adapts accordingly. We evaluate the proposed method on various finite-horizon tasks in planar navigation settings. The empirical results show that the proposed method provides reliable task performance that also improves as the knowledge about the environment enhances.