Biodiversity continues to decline in European agricultural landscapes despite policies that promote biodiversity-friendly farming. While the ecological drivers of the effectiveness of biodiversity-friendly practices are well-studied, less is known about how farmers' attitudes influence biodiversity outcomes. We examined, in eight European countries, whether positive farmer attitudes towards biodiversity are related to higher on-field biodiversity levels and effectiveness of biodiversity-friendly practices. We combined farmer attitude surveys with field inventories of wild plants, bees and spiders in biodiversity-friendly managed and control fields. We found that, in control fields, none of the examined taxa were associated with farmers' biodiversity attitudes. However, in biodiversity-friendly managed fields, both wild bee abundance and species richness increased with a more positive attitude, resulting in a positive relation between farmers' attitude and the effectiveness of biodiversity-friendly management. This relationship was mainly driven by farmers' perceived necessity of considering biodiversity in their farm management, rather than by general biodiversity attitudes. For wild plants and spiders, no relationship between attitude and effectiveness was observed, possibly because these taxa are less conspicuous and less positively perceived. We additionally found that farmers' attitude was positively related to the diversity of biodiversity-friendly practices implemented on farms. Our results suggest that, while agronomical and economic motivations predominate in conventionally managed fields, farmers' biodiversity attitudes can play a role in both the uptake and the effectiveness of biodiversity-friendly practices, although the latter is taxon-dependent. To improve the effectiveness of biodiversity-friendly practices, our findings stress the importance of addressing the social and motivational drivers of farmers.