Forest disturbances are expected to increase as a result of climate change. Thus, it is important to increase the resistance of forests by means of climate adaptation. To examine how effective a voluntary governance approach may be to encourage climate adaptation, this study investigates change in the level of adaptation among private forest owners in Sweden based on survey data from 2014 (n = 1482) and 2018 (n = 1251). In addition, the legitimacy of governance from the owners' perspective is examined by analyzing acceptance of different governance modes in 2018. Results reveal that climate adaptation increased over time and this change can be traced to the strengthening of biodiversity values among the owners. Furthermore, the owners displayed high acceptance of voluntarism and were opposed to detailed regulations. Owners with higher levels of climate adaptation displayed higher acceptance of voluntarism than owners with lower levels and this result remain when structural factors (e.g., size of forest) and management orientations (e.g., certification) are controlled for. Yet, also those with lower adaptation levels preferred voluntarism to detailed regulations. The study indicates that climate adaptation has the potential to increase in a context where adaptation is voluntary, and this approach is legitimate to the forest owners. Nevertheless, whether the increase in adaptation is rapid enough and issues of external legitimacy are important to consider.