This study investigates the expression of modality in Kanakanavu, a critically endangered Formosan language spoken in southern Taiwan. We demonstrate that the language shows two independent systems of modality that are distinguished based on both semantic and formal properties. On the one hand, there are three modal expressions of possibility that semantically involve three paths of sense extension, and are morphosyntactically associated with three types of verb serialization. On the other hand, the language exhibits an epistemic-evidential system that involves four speaker-oriented adverbial expressions that occur in clause-initial position. We further show that there are variations among five Formosan languages concerning the sense extension of possibility expressions, and that a unique case of necessity—anticipative necessity—is shared by Kanakanavu, Tsou, Mayrinax Atayal, and Seediq. Typologically, the modal system in Kanakanavu shows a lack of alignment between event modality and epistemic modality, the latter exhibiting a stronger bond with evidentiality. This observed phenomenon is in sharp contrast to commonly found European/English-type modal systems in which the event-epistemic overlap is prevalent.