This dissertation examines human interaction with the nonhuman environment on the island of Taiwan, highlighting the role of the mountains and the sea in the experience of being Taiwanese. The topic relates to the ongoing process of “Taiwanisation”, which refers to a general shift from a China-centric to a Taiwan centric perspective among the island’s population. Taiwanisation has emerged in tandem with Taiwan’s political liberalisation, and it has typically been analysed through the concepts of Taiwanese subjectivity, consciousness, and identity. Based on a posthumanist framework that rejects the modern dualisms of mind-body, subject-object, and society-nature, this study argues that Taiwanisation can also be viewed as a process of ecological adaptation to the island environment. Such adaptation counters the environmental alienation that has characterised Taiwan since its seventeenth-century colonisation by the Han Chinese. Due to the anthropocentric tenets of “Sinitic modernity” and the vagaries of the Chinese Civil War, the Han society in Taiwan developed a mainland-oriented mentality that constrains perceptions and experiences of the island to this day. This study investigates the ensuing dynamics of alienation and adaptation by taking outdoor activities in the mountains and the sea as examples of “ecological Taiwanisation”. Through a thematic analysis based on historical sources and ethnographic fieldwork with Han Taiwanese hikers and surfers, this study argues that outdoor activities challenge the mainland mentality by facilitating discoveries about the island environment, its indigenous peoples, and its Japanese colonial heritage. Meanwhile, impediments to such discoveries continue to arise from alienating elements associated with Confucian values, Han folk beliefs, and the military-political legacy of the Chinese Civil War. The findings suggest that bodily activity in the natural environment can build awareness and attenuate perspectival biases by highlighting the role of marginalised peoples and places. The persistence of alienating influences suggests that this practical process would benefit from the wider adoption of a corresponding theoretical orientation. The study thus indicates the usefulness of posthumanist approaches in addressing the oversights of anthropocentric modernity.