French and EU policies have been only partially successful in promoting the restoration of groundwater quality. The currently proposed measures are scientifically valid but ineffective in encouraging farmers to change their practices over the longer term. Participatory approaches have been developed for co-designing scenarios at cropping system or catchment area level, to improve groundwater quality. Farmers are one of several types of stakeholders who make contributions in this respect. In this context, we propose a similar participatory approach, although with two key differences: only farmers take part in the co-design process, and a farm-scale systemic perspective is applied. Our method, inspired by co-development, involves five steps, including groundwater quality pressure assessment. Within this method, we generate farmer-to-farmer suggestions aimed at improving farm management from an economic, social, and environmental perspective, with an emphasis on reducing pollution in catchments. The co-design groundwater-friendly farm management combines re-designed elements (e.g., changing agricultural practices or cropping systems or machinery or labor) that are consistent with the project specified by the farmer and that simultaneously decrease pressure on groundwater quality. We tested our method using two groups of farmers from southeastern France, located in areas concerned by groundwater quality issues related to nitrate and pesticide pollution. Our results show that our method based on farmer-to-farmer exchanges with a systemic approach constitutes an interesting and viable solution. In the months following the co-design process, the farmers in the test groups implemented some of the innovations suggested by their peers, thus creating a new groundwater-friendly farm management. This approach could be used in regions with other environmental challenges since the ultimate goal is to encourage sustainable farming practices. However, in the proposed methodology, the knowledge provided only by farmers might be too homogeneous, thus limiting the scope of changes in farming practices.