||Nature-based tourism in protected areas has grown worldwide in recent years, but excessive use of natural areas can result in their degradation or loss. Visitor management should be based on proper monitoring data to achieve quality experiences for visitors without damaging nature resources. Visitor data are typically collected through field surveys, but budget and human resource constraints can limit the spatiotemporal resolution of survey data. Geotagged photos and messages posted on social media by visitors have attracted attention as useful sources of information with high spatiotemporal resolution. Previous studies, however, have raised concerns that biases in social media data arising from the sociodemographic attributes of posters can create challenges in determining who and what the social media data represent, and in interpreting this data in a reliable way. The present study focused on nationality bias, i.e., differences between the nationality of actual visitors versus the nationality of those visitors who post on social media. Nationality bias can arise due to significant differences in the use of social media from country to country (including the proportion of the population using social media). Cultural and values differences between countries can also greatly influence visitor behavior, and these gaps may lead to over- or under-estimation of visitors from specific countries or regions. However, nationality bias is not fully understood because few studies have explicitly considered it until now (e.g., Heikinheimo et al., 2017; Sinclair et al., 2020). In addition, the previous studies were confined to protected areas where the majority of visitors were from Europe.