National parks are popular destinations for tourism and recreation. Managing visitors to such parks is important, including assessing where people go and when, and what biodiversity they value. Traditional on-site methods are starting to be complemented with online user-created content, and this could include data from iNaturalist, a massively popular (>8 million users) biodiversity citizen science platform. To assess if iNaturalist data can provide insights into visitation to parks with a wide range of types of landscapes, metadata consisting of 96,070 observations on iNaturalist posted by 5378 people, posting geolocated in the National Parks Network in Spain (2015–2024) were downloaded and analyzed. Visitors' spatial distribution on and off-trails varied among the 16 national parks, with a strong positive correlation with the total distance of trails per national park (Spearman's rs = 0.94, p = < 0.001). Monthly patterns of visitation based on iNaturalist metadata were similar to official records (Spearman's rs = 0.69, p = 0.005) as was the spatial distribution of observations per national park was correlated to the relative number of visits per km2 (Spearman's rs = 0.51, p = 0.004). Plants (52 %), birds (22 %) and insects (12 %) were the most common biodiversity recorded on iNaturalist, including national parks in mainland Spain and in the Canary Islands. Although there are some important limitations with online user-created content, the utility of iNaturalist can be extended to complementing on-site visitation assessment methods, helping managers and researchers to expand the types of visitor data they can access, including spatiotemporal patterns of visitation and biodiversity values at a national scale.